On the slow and successive appearance of new species -- On their differentrates of change -- Species once lost do not reappear -- Groups of speciesfollow the same general rules in their appearance and disappearance as dosingle species -- On extinction -- On simultaneous changes in the forms oflife throughout the world -- On the affinities of extinct species to eachother and to living species -- On the state of development of ancient forms-- On the succession of the same types within the same areas -- Summary ofpreceding and present chapters.

Let us now see whether the several facts and laws relating to thegeological succession of organic beings accord best with the common view ofthe immutability of species, or with that of their slow and gradualmodification, through variation and natural selection.

New species have appeared very slowly, one after another, both on the landand in the waters. Lyell has shown that it is hardly possible to resistthe evidence on this head in the case of the several tertiary stages; andevery year tends to fill up the blanks between the stages, and to make theproportion between the lost and existing forms more gradual. In some ofthe most recent beds, though undoubtedly of high antiquity if measured byyears, only one or two species are extinct, and only one or two are new,having appeared there for the first time, either locally, or, as far as weknow, on the face of the earth. The secondary formations are more broken;but, as Bronn has remarked, neither the appearance nor disappearance of themany species embedded in each formation has been simultaneous.

Species belonging to different genera and classes have not changed at thesame rate, or in the same degree. In the older tertiary beds a few livingshells may still be found in the midst of a multitude of extinct forms. Falconer has given a striking instance of a similar fact, for an existingcrocodile is associated with many lost mammals and reptiles in thesub-Himalayan deposits. The Silurian Lingula differs but little from theliving species of this genus; whereas most of the other Silurian Molluscsand all the Crustaceans have changed greatly. The productions of the landseem to have changed at a quicker rate than those of the sea, of which astriking instance has been observed in Switzerland. There is some reasonto believe that organisms high in the scale, change more quickly than thosethat are low: though there are exceptions to this rule. The amount oforganic change, as Pictet has remarked, is not the same in each successiveso-called formation. Yet if we compare any but the most closely relatedformations, all the species will be found to have undergone some change. When a species has once disappeared from the face of the earth, we have noreason to believe that the same identical form ever reappears. Thestrongest apparent exception to this latter rule is that of the so-called"colonies" of M. Barrande, which intrude for a period in the midst of anolder formation, and then allow the pre-existing fauna to reappear; butLyell's explanation, namely, that it is a case of temporary migration froma distinct geographical province, seems satisfactory.

These several facts accord well with our theory, which includes no fixedlaw of development, causing all the inhabitants of an area to changeabruptly, or simultaneously, or to an equal degree. The process ofmodification must be slow, and will generally affect only a few species atthe same time; for the variability of each species is independent of thatof all others. Whether such variations or individual differences as mayarise will be accumulated through natural selection in a greater or lessdegree, thus causing a greater or less amount of permanent modification, will depend on many complex contingencies--on the variations being of abeneficial nature, on the freedom of intercrossing, on the slowly changingphysical conditions of the country, on the immigration of new colonists,and on the nature of the other inhabitants with which the varying speciescome into competition. Hence it is by no means surprising that one speciesshould retain the same identical form much longer than others; or, ifchanging, should change in a less degree. We find similar relationsbetween the existing inhabitants of distinct countries; for instance, theland-shells and coleopterous insects of Madeira have come to differconsiderably from their nearest allies on the continent of Europe, whereasthe marine shells and birds have remained unaltered. We can perhapsunderstand the apparently quicker rate of change in terrestrial and in morehighly organised productions compared with marine and lower productions, bythe more complex relations of the higher beings to their organic andinorganic conditions of life, as explained in a former chapter. When manyof the inhabitants of any area have become modified and improved, we canunderstand, on the principle of competition, and from the all-importantrelations of organism to organism in the struggle for life, that any formwhich did not become in some degree modified and improved, would be liableto extermination. Hence, we see why all the species in the same region doat last, if we look to long enough intervals of time, become modified; forotherwise they would become extinct.

In members of the same class the average amount of change, during long andequal periods of time, may, perhaps, be nearly the same; but as theaccumulation of enduring formations, rich in fossils, depends on greatmasses of sediment being deposited on subsiding areas, our formations havebeen almost necessarily accumulated at wide and irregularly intermittentintervals of time; consequently the amount of organic change exhibited bythe fossils embedded in consecutive formations is not equal. Eachformation, on this view, does not mark a new and complete act of creation,but only an occasional scene, taken almost at hazard, in an ever slowlychanging drama.

We can clearly understand why a species when once lost should neverreappear, even if the very same conditions of life, organic and inorganic,should recur. For though the offspring of one species might be adapted(and no doubt this has occurred in innumerable instances) to fill the placeof another species in the economy of nature, and thus supplant it; yet thetwo forms--the old and the new--would not be identically the same; for bothwould almost certainly inherit different characters from their distinctprogenitors; and organisms already differing would vary in a differentmanner. For instance, it is possible, if all our fantail-pigeons weredestroyed, that fanciers might make a new breed hardly distinguishable fromthe present breed; but if the parent rock-pigeon were likewise destroyed,and under nature we have every reason to believe that parent forms aregenerally supplanted and exterminated by their improved offspring, it isincredible that a fantail, identical with the existing breed, could beraised from any other species of pigeon, or even from any other wellestablished race of the domestic pigeon, for the successive variationswould almost certainly be in some degree different, and the newly-formedvariety would probably inherit from its progenitor some characteristicdifferences.

Groups of species, that is, genera and families, follow the same generalrules in their appearance and disappearance as do single species, changingmore or less quickly, and in a greater or lesser degree. A group, when ithas once disappeared, never reappears; that is, its existence, as long asit lasts, is continuous. I am aware that there are some apparentexceptions to this rule, but the exceptions are surprisingly few, so fewthat E. Forbes, Pictet, and Woodward (though all strongly opposed to suchviews as I maintain) admit its truth; and the rule strictly accords withthe theory. For all the species of the same group, however long it mayhave lasted, are the modified descendants one from the other, and all froma common progenitor. In the genus Lingula, for instance, the species whichhave successively appeared at all ages must have been connected by anunbroken series of generations, from the lowest Silurian stratum to thepresent day.

We have seen in the last chapter that whole groups of species sometimesfalsely appear to have been abruptly developed; and I have attempted togive an explanation of this fact, which if true would be fatal to my views. But such cases are certainly exceptional; the general rule being a gradualincrease in number, until the group reaches its maximum, and then, sooneror later, a gradual decrease. If the number of the species included withina genus, or the number of the genera within a family, be represented by avertical line of varying thickness, ascending through the successivegeological formations, in which the species are found, the line willsometimes falsely appear to begin at its lower end, not in a sharp point,but abruptly; it then gradually thickens upwards, often keeping of equalthickness for a space, and ultimately thins out in the upper beds, markingthe decrease and final extinction of the species. This gradual increase innumber of the species of a group is strictly conformable with the theory;for the species of the same genus, and the genera of the same family, canincrease only slowly and progressively; the process of modification and theproduction of a number of allied forms necessarily being a slow and gradualprocess, one species first giving rise to two or three varieties, thesebeing slowly converted into species, which in their turn produce by equallyslow steps other varieties and species, and so on, like the branching of agreat tree from a single stem, till the group becomes large.

ON EXTINCTION.

We have as yet only spokesn incidentally of the disappearance of speciesand of groups of species. On the theory of natural selection, theextinction of old forms and the production of new and improved forms areintimately connected together. The old notion of all the inhabitants ofthe earth having been swept away by catastrophes at successive periods isvery generally given up, even by those geologists, as Elie de Beaumont,Murchison, Barrande, etc., whose general views would naturally lead them tothis conclusion. On the contrary, we have every reason to believe, fromthe study of the tertiary formations, that species and groups of speciesgradually disappear, one after another, first from one spot, then fromanother, and finally from the world. In some few cases, however, as by thebreaking of an isthmus and the consequent irruption of a multitude of newinhabitants into an adjoining sea, or by the final subsidence of an island,the process of extinction may have been rapid. Both single species andwhole groups of species last for very unequal periods; some groups, as wehave seen, have endured from the earliest known dawn of life to the presentday; some have disappeared before the close of the palaeozoic period. Nofixed law seems to determine the length of time during which any singlespecies or any single genus endures. There is reason to believe that theextinction of a whole group of species is generally a slower process thantheir production: if their appearance and disappearance be represented, asbefore, by a vertical line of varying thickness the line is found to tapermore gradually at its upper end, which marks the progress of extermination,than at its lower end, which marks the first appearance and the earlyincrease in number of the species. In some cases, however, theextermination of whole groups, as of ammonites, towards the close of thesecondary period, has been wonderfully sudden.

The extinction of species has been involved in the most gratuitous mystery. Some authors have even supposed that, as the individual has a definitelength of life, so have species a definite duration. No one can havemarvelled more than I have done at the extinction of species. When I foundin La Plata the tooth of a horse embedded with the remains of Mastodon,Megatherium, Toxodon and other extinct monsters, which all co-existed withstill living shells at a very late geological period, I was filled withastonishment; for, seeing that the horse, since its introduction by theSpaniards into South America, has run wild over the whole country and hasincreased in numbers at an unparalleled rate, I asked myself what could sorecently have exterminated the former horse under conditions of lifeapparently so favourable. But my astonishment was groundless. ProfessorOwen soon perceived that the tooth, though so like that of the existinghorse, belonged to an extinct species. Had this horse been still living,but in some degree rare, no naturalist would have felt the least surpriseat its rarity; for rarity is the attribute of a vast number of species ofall classes, in all countries. If we ask ourselves why this or thatspecies is rare, we answer that something is unfavourable in its conditionsof life; but what that something is, we can hardly ever tell. On thesupposition of the fossil horse still existing as a rare species, we mighthave felt certain, from the analogy of all other mammals, even of theslow-breeding elephant, and from the history of the naturalisation of thedomestic horse in South America, that under more favourable conditions itwould in a very few years have stocked the whole continent. But we couldnot have told what the unfavourable conditions were which checked itsincrease, whether some one or several contingencies, and at what period ofthe horse's life, and in what degree they severally acted. If theconditions had gone on, however slowly, becoming less and less favourable,we assuredly should not have perceived the fact, yet the fossil horse wouldcertainly have become rarer and rarer, and finally extinct--its place beingseized on by some more successful competitor.

It is most difficult always to remember that the increase of every livingcreature is constantly being checked by unperceived hostile agencies; andthat these same unperceived agencies are amply sufficient to cause rarity,and finally extinction. So little is this subject understood, that I haveheard surprise repeatedly expressed at such great monsters as the Mastodonand the more ancient Dinosaurians having become extinct; as if mere bodilystrength gave victory in the battle of life. Mere size, on the contrary,would in some cases determine, as has been remarked by Owen, quickerextermination, from the greater amount of requisite food. Before maninhabited India or Africa, some cause must have checked the continuedincrease of the existing elephant. A highly capable judge, Dr. Falconer,believes that it is chiefly insects which, from incessantly harassing andweakening the elephant in India, check its increase; and this was Bruce'sconclusion with respect to the African elephant in Abyssinia. It iscertain that insects and blood-sucking bats determine the existence of thelarger naturalised quadrupeds in several parts of South America.

We see in many cases in the more recent tertiary formations that rarityprecedes extinction; and we know that this has been the progress of eventswith those animals which have been exterminated, either locally or wholly,through man's agency. I may repeat what I published in 1845, namely, thatto admit that species generally become rare before they become extinct--tofeel no surprise at the rarity of a species, and yet to marvel greatly whenthe species ceases to exist, is much the same as to admit that sickness inthe individual is the forerunner of death--to feel no surprise at sickness,but, when the sick man dies, to wonder and to suspect that he died by somedeed of violence.

The theory of natural selection is grounded on the belief that each newvariety and ultimately each new species, is produced and maintained byhaving some advantage over those with which it comes into competition; andthe consequent extinction of less-favoured forms almost inevitably follows. It is the same with our domestic productions: when a new and slightlyimproved variety has been raised, it at first supplants the less improvedvarieties in the same neighbourhood; when much improved it is transportedfar and near, like our short-horn cattle, and takes the place of otherbreeds in other countries. Thus the appearance of new forms and thedisappearance of old forms, both those naturally and artificially produced,are bound together. In flourishing groups, the number of new specificforms which have been produced within a given time has at some periodsprobably been greater than the number of the old specific forms which havebeen exterminated; but we know that species have not gone on indefinitelyincreasing, at least during the later geological epochs, so that, lookingto later times, we may believe that the production of new forms has causedthe extinction of about the same number of old forms.

The competition will generally be most severe, as formerly explained andillustrated by examples, between the forms which are most like each otherin all respects. Hence the improved and modified descendants of a specieswill generally cause the extermination of the parent-species; and if manynew forms have been developed from any one species, the nearest allies ofthat species, i.e. the species of the same genus, will be the most liableto extermination. Thus, as I believe, a number of new species descendedfrom one species, that is a new genus, comes to supplant an old genus,belonging to the same family. But it must often have happened that a newspecies belonging to some one group has seized on the place occupied by aspecies belonging to a distinct group, and thus have caused itsextermination. If many allied forms be developed from the successfulintruder, many will have to yield their places; and it will generally bethe allied forms, which will suffer from some inherited inferiority incommon. But whether it be species belonging to the same or to a distinctclass, which have yielded their places to other modified and improvedspecies, a few of the sufferers may often be preserved for a long time,from being fitted to some peculiar line of life, or from inhabiting somedistant and isolated station, where they will have escaped severecompetition. For instance, some species of Trigonia, a great genus ofshells in the secondary formations, survive in the Australian seas; and afew members of the great and almost extinct group of Ganoid fishes stillinhabit our fresh waters. Therefore, the utter extinction of a group isgenerally, as we have seen, a slower process than its production.

With respect to the apparently sudden extermination of whole families ororders, as of Trilobites at the close of the palaeozoic period, and ofAmmonites at the close of the secondary period, we must remember what hasbeen already said on the probable wide intervals of time between ourconsecutive formations; and in these intervals there may have been muchslow extermination. Moreover, when, by sudden immigration or by unusuallyrapid development, many species of a new group have taken possession of anarea, many of the older species will have been exterminated in acorrespondingly rapid manner; and the forms which thus yield their placeswill commonly be allied, for they will partake of the same inferiority incommon.

Thus, as it seems to me, the manner in which single species and wholegroups of species become extinct accords well with the theory of naturalselection. We need not marvel at extinction; if we must marvel, let it beat our presumption in imagining for a moment that we understand the manycomplex contingencies on which the existence of each species depends. Ifwe forget for an instant that each species tends to increase inordinately,and that some check is always in action, yet seldom perceived by us, thewhole economy of nature will be utterly obscured. Whenever we canprecisely say why this species is more abundant in individuals than that;why this species and not another can be naturalised in a given country;then, and not until then, we may justly feel surprise why we cannot accountfor the extinction of any particular species or group of species.

ON THE FORMS OF LIFE CHANGING ALMOST SIMULTANEOUSLY THROUGHOUT THE WORLD.

Scarcely any palaeontological discovery is more striking than the fact thatthe forms of life change almost simultaneously throughout the world. Thusour European Chalk formation can be recognised in many distant regions,under the most different climates, where not a fragment of the mineralchalk itself can be found; namely, in North America, in equatorial SouthAmerica, in Tierra del Fuego, at the Cape of Good Hope, and in thepeninsula of India. For at these distant points, the organic remains incertain beds present an unmistakable resemblance to those of the Chalk. Itis not that the same species are met with; for in some cases not onespecies is identically the same, but they belong to the same families,genera, and sections of genera, and sometimes are similarly characterisedin such trifling points as mere superficial sculpture. Moreover, otherforms, which are not found in the Chalk of Europe, but which occur in theformations either above or below, occur in the same order at these distantpoints of the world. In the several successive palaeozoic formations ofRussia, Western Europe and North America, a similar parallelism in theforms of life has been observed by several authors; so it is, according toLyell, with the European and North American tertiary deposits. Even if thefew fossil species which are common to the Old and New Worlds were keptwholly out of view, the general parallelism in the successive forms oflife, in the palaeozoic and tertiary stages, would still be manifest, andthe several formations could be easily correlated.

These observations, however, relate to the marine inhabitants of the world: we have not sufficient data to judge whether the productions of the landand of fresh water at distant points change in the same parallel manner. We may doubt whether they have thus changed: if the Megatherium, Mylodon,Macrauchenia, and Toxodon had been brought to Europe from La Plata, withoutany information in regard to their geological position, no one would havesuspected that they had co-existed with sea-shells all still living; but asthese anomalous monsters co-existed with the Mastodon and Horse, it mightat least have been inferred that they had lived during one of the latertertiary stages.

When the marine forms of life are spoken of as having changedsimultaneously throughout the world, it must not be supposed that thisexpression relates to the same year, or even to the same century, or eventhat it has a very strict geological sense; for if all the marine animalsnow living in Europe, and all those that lived in Europe during thepleistocene period (a very remote period as measured by years, includingthe whole glacial epoch) were compared with those now existing in SouthAmerica or in Australia, the most skilful naturalist would hardly be ableto say whether the present or the pleistocene inhabitants of Europeresembled most closely those of the southern hemisphere. So, again,several highly competent observers maintain that the existing productionsof the United States are more closely related to those which lived inEurope during certain late tertiary stages, than to the present inhabitantsof Europe; and if this be so, it is evident that fossiliferous beds nowdeposited on the shores of North America would hereafter be liable to beclassed with somewhat older European beds. Nevertheless, looking to aremotely future epoch, there can be little doubt that all the more modernMARINE formations, namely, the upper pliocene, the pleistocene and strictlymodern beds of Europe, North and South America, and Australia, fromcontaining fossil remains in some degree allied, and from not includingthose forms which are found only in the older underlying deposits, would becorrectly ranked as simultaneous in a geological sense.

The fact of the forms of life changing simultaneously in the above largesense, at distant parts of the world, has greatly struck those admirableobservers, MM. de Verneuil and d'Archiac. After referring to theparallelism of the palaeozoic forms of life in various parts of Europe,they add, "If struck by this strange sequence, we turn our attention toNorth America, and there discover a series of analogous phenomena, it willappear certain that all these modifications of species, their extinction,and the introduction of new ones, cannot be owing to mere changes in marinecurrents or other causes more or less local and temporary, but depend ongeneral laws which govern the whole animal kingdom." M. Barrande has madeforcible remarks to precisely the same effect. It is, indeed, quite futileto look to changes of currents, climate, or other physical conditions, asthe cause of these great mutations in the forms of life throughout theworld, under the most different climates. We must, as Barrande hasremarked, look to some special law. We shall see this more clearly when wetreat of the present distribution of organic beings, and find how slight isthe relation between the physical conditions of various countries and thenature of their inhabitants.

This great fact of the parallel succession of the forms of life throughoutthe world, is explicable on the theory of natural selection. New speciesare formed by having some advantage over older forms; and the forms, whichare already dominant, or have some advantage over the other forms in theirown country, give birth to the greatest number of new varieties orincipient species. We have distinct evidence on this head, in the plantswhich are dominant, that is, which are commonest and most widely diffused,producing the greatest number of new varieties. It is also natural thatthe dominant, varying and far-spreading species, which have alreadyinvaded, to a certain extent, the territories of other species, should bethose which would have the best chance of spreading still further, and ofgiving rise in new countries to other new varieties and species. Theprocess of diffusion would often be very slow, depending on climatal andgeographical changes, on strange accidents, and on the gradualacclimatization of new species to the various climates through which theymight have to pass, but in the course of time the dominant forms wouldgenerally succeed in spreading and would ultimately prevail. The diffusionwould, it is probable, be slower with the terrestrial inhabitants ofdistinct continents than with the marine inhabitants of the continuous sea. We might therefore expect to find, as we do find, a less strict degree ofparallelism in the succession of the productions of the land than withthose of the sea.

Thus, as it seems to me, the parallel, and, taken in a large sense,simultaneous, succession of the same forms of life throughout the world,accords well with the principle of new species having been formed bydominant species spreading widely and varying; the new species thusproduced being themselves dominant, owing to their having had someadvantage over their already dominant parents, as well as over otherspecies; and again spreading, varying, and producing new forms. The oldforms which are beaten and which yield their places to the new andvictorious forms, will generally be allied in groups, from inheriting someinferiority in common; and, therefore, as new and improved groups spreadthroughout the world, old groups disappear from the world; and thesuccession of forms everywhere tends to correspond both in their firstappearance and final disappearance.

There is one other remark connected with this subject worth making. I havegiven my reasons for believing that most of our great formations, rich infossils, were deposited during periods of subsidence; and that blankintervals of vast duration, as far as fossils are concerned, occurredduring the periods when the bed of the sea was either stationary or rising,and likewise when sediment was not thrown down quickly enough to embed andpreserve organic remains. During these long and blank intervals I supposethat the inhabitants of each region underwent a considerable amount ofmodification and extinction, and that there was much migration from otherparts of the world. As we have reason to believe that large areas areaffected by the same movement, it is probable that strictly contemporaneousformations have often been accumulated over very wide spaces in the samequarter of the world; but we are very far from having any right to concludethat this has invariably been the case, and that large areas haveinvariably been affected by the same movements. When two formations havebeen deposited in two regions during nearly, but not exactly, the sameperiod, we should find in both, from the causes explained in the foregoingparagraphs, the same general succession in the forms of life; but thespecies would not exactly correspond; for there will have been a littlemore time in the one region than in the other for modification, extinction,and immigration.

I suspect that cases of this nature occur in Europe. Mr. Prestwich, in hisadmirable Memoirs on the eocene deposits of England and France, is able todraw a close general parallelism between the successive stages in the twocountries; but when he compares certain stages in England with those inFrance, although he finds in both a curious accordance in the numbers ofthe species belonging to the same genera, yet the species themselves differin a manner very difficult to account for considering the proximity of thetwo areas, unless, indeed, it be assumed that an isthmus separated two seasinhabited by distinct, but contemporaneous faunas. Lyell has made similarobservations on some of the later tertiary formations. Barrande, also,shows that there is a striking general parallelism in the successiveSilurian deposits of Bohemia and Scandinavia; nevertheless he finds asurprising amount of difference in the species. If the several formationsin these regions have not been deposited during the same exact periods--aformation in one region often corresponding with a blank interval in theother--and if in both regions the species have gone on slowly changingduring the accumulation of the several formations and during the longintervals of time between them; in this case the several formations in thetwo regions could be arranged in the same order, in accordance with thegeneral succession of the forms of life, and the order would falsely appearto be strictly parallel; nevertheless the species would not all be the samein the apparently corresponding stages in the two regions.

ON THE AFFINITIES OF EXTINCT SPECIES TO EACH OTHER, AND TO LIVING FORMS.

Let us now look to the mutual affinities of extinct and living species. All fall into a few grand classes; and this fact is at once explained onthe principle of descent. The more ancient any form is, the more, as ageneral rule, it differs from living forms. But, as Buckland long agoremarked, extinct species can all be classed either in still existinggroups, or between them. That the extinct forms of life help to fill upthe intervals between existing genera, families, and orders, is certainlytrue; but as this statement has often been ignored or even denied, it maybe well to make some remarks on this subject, and to give some instances. If we confine our attention either to the living or to the extinct speciesof the same class, the series is far less perfect than if we combine bothinto one general system. In the writings of Professor Owen we continuallymeet with the expression of generalised forms, as applied to extinctanimals; and in the writings of Agassiz, of prophetic or synthetic types;and these terms imply that such forms are, in fact, intermediate orconnecting links. Another distinguished palaeontologist, M. Gaudry, hasshown in the most striking manner that many of the fossil mammalsdiscovered by him in Attica serve to break down the intervals betweenexisting genera. Cuvier ranked the Ruminants and Pachyderms as two of themost distinct orders of mammals; but so many fossil links have beendisentombed that Owen has had to alter the whole classification, and hasplaced certain Pachyderms in the same sub-order with ruminants; forexample, he dissolves by gradations the apparently wide interval betweenthe pig and the camel. The Ungulata or hoofed quadrupeds are now dividedinto the even-toed or odd-toed divisions; but the Macrauchenia of SouthAmerica connects to a certain extent these two grand divisions. No onewill deny that the Hipparion is intermediate between the existing horse andcertain other ungulate forms. What a wonderful connecting link in thechain of mammals is the Typotherium from South America, as the name givento it by Professor Gervais expresses, and which cannot be placed in anyexisting order. The Sirenia form a very distinct group of the mammals, andone of the most remarkable peculiarities in existing dugong and lamentin isthe entire absence of hind limbs, without even a rudiment being left; butthe extinct Halitherium had, according to Professor Flower, an ossifiedthigh-bone "articulated to a well-defined acetabulum in the pelvis," and itthus makes some approach to ordinary hoofed quadrupeds, to which theSirenia are in other respects allied. The cetaceans or whales are widelydifferent from all other mammals, but the tertiary Zeuglodon and Squalodon,which have been placed by some naturalists in an order by themselves, areconsidered by Professor Huxley to be undoubtedly cetaceans, "and toconstitute connecting links with the aquatic carnivora."

Even the wide interval between birds and reptiles has been shown by thenaturalist just quoted to be partially bridged over in the most unexpectedmanner, on the one hand, by the ostrich and extinct Archeopteryx, and onthe other hand by the Compsognathus, one of the Dinosaurians--that groupwhich includes the most gigantic of all terrestrial reptiles. Turning tothe Invertebrata, Barrande asserts, a higher authority could not be named,that he is every day taught that, although palaeozoic animals can certainlybe classed under existing groups, yet that at this ancient period thegroups were not so distinctly separated from each other as they now are.

Some writers have objected to any extinct species, or group of species,being considered as intermediate between any two living species, or groupsof species. If by this term it is meant that an extinct form is directlyintermediate in all its characters between two living forms or groups, theobjection is probably valid. But in a natural classification many fossilspecies certainly stand between living species, and some extinct generabetween living genera, even between genera belonging to distinct families. The most common case, especially with respect to very distinct groups, suchas fish and reptiles, seems to be that, supposing them to be distinguishedat the present day by a score of characters, the ancient members areseparated by a somewhat lesser number of characters, so that the two groupsformerly made a somewhat nearer approach to each other than they now do.

It is a common belief that the more ancient a form is, by so much the moreit tends to connect by some of its characters groups now widely separatedfrom each other. This remark no doubt must be restricted to those groupswhich have undergone much change in the course of geological ages; and itwould be difficult to prove the truth of the proposition, for every now andthen even a living animal, as the Lepidosiren, is discovered havingaffinities directed towards very distinct groups. Yet if we compare theolder Reptiles and Batrachians, the older Fish, the older Cephalopods, andthe eocene Mammals, with the recent members of the same classes, we mustadmit that there is truth in the remark.

Let us see how far these several facts and inferences accord with thetheory of descent with modification. As the subject is somewhat complex, Imust request the reader to turn to the diagram in the fourth chapter. Wemay suppose that the numbered letters in italics represent genera, and thedotted lines diverging from them the species in each genus. The diagram ismuch too simple, too few genera and too few species being given, but thisis unimportant for us. The horizontal lines may represent successivegeological formations, and all the forms beneath the uppermost line may beconsidered as extinct. The three existing genera, a14, q14, p14, will forma small family; b14 and f14, a closely allied family or subfamily; and o14,i14, m14, a third family. These three families, together with the manyextinct genera on the several lines of descent diverging from the parentform (A) will form an order; for all will have inherited something incommon from their ancient progenitor. On the principle of the continuedtendency to divergence of character, which was formerly illustrated by thisdiagram, the more recent any form is the more it will generally differ fromits ancient progenitor. Hence, we can understand the rule that the mostancient fossils differ most from existing forms. We must not, however,assume that divergence of character is a necessary contingency; it dependssolely on the descendants from a species being thus enabled to seize onmany and different places in the economy of nature. Therefore it is quitepossible, as we have seen in the case of some Silurian forms, that aspecies might go on being slightly modified in relation to its slightlyaltered conditions of life, and yet retain throughout a vast period thesame general characteristics. This is represented in the diagram by theletter F14.

All the many forms, extinct and recent, descended from (A), make, as beforeremarked, one order; and this order, from the continued effects ofextinction and divergence of character, has become divided into severalsub-families and families, some of which are supposed to have perished atdifferent periods, and some to have endured to the present day.

By looking at the diagram we can see that if many of the extinct formssupposed to be embedded in the successive formations, were discovered atseveral points low down in the series, the three existing families on theuppermost line would be rendered less distinct from each other. If, forinstance, the genera a1, a5, a10, f8, m3, m6, m9, were disinterred, thesethree families would be so closely linked together that they probably wouldhave to be united into one great family, in nearly the same manner as hasoccurred with ruminants and certain pachyderms. Yet he who objected toconsider as intermediate the extinct genera, which thus link together theliving genera of three families, would be partly justified, for they areintermediate, not directly, but only by a long and circuitous coursethrough many widely different forms. If many extinct forms were to bediscovered above one of the middle horizontal lines or geologicalformations--for instance, above No. VI.--but none from beneath this line,then only two of the families (those on the left hand a14, etc., and b14,etc.) would have to be united into one; and there would remain two familieswhich would be less distinct from each other than they were before thediscovery of the fossils. So again, if the three families formed of eightgenera (a14 to m14), on the uppermost line, be supposed to differ from eachother by half-a-dozen important characters, then the families which existedat a period marked VI would certainly have differed from each other by aless number of characters; for they would at this early stage of descenthave diverged in a less degree from their common progenitor. Thus it comesthat ancient and extinct genera are often in a greater or less degreeintermediate in character between their modified descendants, or betweentheir collateral relations.

Under nature the process will be far more complicated than is representedin the diagram; for the groups will have been more numerous; they will haveendured for extremely unequal lengths of time, and will have been modifiedin various degrees. As we possess only the last volume of the geologicalrecord, and that in a very broken condition, we have no right to expect,except in rare cases, to fill up the wide intervals in the natural system,and thus to unite distinct families or orders. All that we have a right toexpect is, that those groups which have, within known geological periods,undergone much modification, should in the older formations make someslight approach to each other; so that the older members should differ lessfrom each other in some of their characters than do the existing members ofthe same groups; and this by the concurrent evidence of our bestpalaeontologists is frequently the case.

Thus, on the theory of descent with modification, the main facts withrespect to the mutual affinities of the extinct forms of life to each otherand to living forms, are explained in a satisfactory manner. And they arewholly inexplicable on any other view.

On this same theory, it is evident that the fauna during any one greatperiod in the earth's history will be intermediate in general characterbetween that which preceded and that which succeeded it. Thus the specieswhich lived at the sixth great stage of descent in the diagram are themodified offspring of those which lived at the fifth stage, and are theparents of those which became still more modified at the seventh stage;hence they could hardly fail to be nearly intermediate in character betweenthe forms of life above and below. We must, however, allow for the entireextinction of some preceding forms, and in any one region for theimmigration of new forms from other regions, and for a large amount ofmodification during the long and blank intervals between the successiveformations. Subject to these allowances, the fauna of each geologicalperiod undoubtedly is intermediate in character, between the preceding andsucceeding faunas. I need give only one instance, namely, the manner inwhich the fossils of the Devonian system, when this system was firstdiscovered, were at once recognised by palaeontologists as intermediate incharacter between those of the overlying carboniferous and underlyingSilurian systems. But each fauna is not necessarily exactly intermediate,as unequal intervals of time have elapsed between consecutive formations.

It is no real objection to the truth of the statement that the fauna ofeach period as a whole is nearly intermediate in character between thepreceding and succeeding faunas, that certain genera offer exceptions tothe rule. For instance, the species of mastodons and elephants, whenarranged by Dr. Falconer in two series--in the first place according totheir mutual affinities, and in the second place according to their periodsof existence--do not accord in arrangement. The species extreme incharacter are not the oldest or the most recent; nor are those which areintermediate in character, intermediate in age. But supposing for aninstant, in this and other such cases, that the record of the firstappearance and disappearance of the species was complete, which is far fromthe case, we have no reason to believe that forms successively producednecessarily endure for corresponding lengths of time. A very ancient formmay occasionally have lasted much longer than a form elsewhere subsequentlyproduced, especially in the case of terrestrial productions inhabitingseparated districts. To compare small things with great; if the principalliving and extinct races of the domestic pigeon were arranged in serialaffinity, this arrangement would not closely accord with the order in timeof their production, and even less with the order of their disappearance;for the parent rock-pigeon still lives; and many varieties between therock-pigeon and the carrier have become extinct; and carriers which areextreme in the important character of length of beak originated earlierthan short-beaked tumblers, which are at the opposite end of the series inthis respect.

Closely connected with the statement, that the organic remains from anintermediate formation are in some degree intermediate in character, is thefact, insisted on by all palaeontologists, that fossils from twoconsecutive formations are far more closely related to each other, than arethe fossils from two remote formations. Pictet gives as a well-knowninstance, the general resemblance of the organic remains from the severalstages of the Chalk formation, though the species are distinct in eachstage. This fact alone, from its generality, seems to have shakenProfessor Pictet in his belief in the immutability of species. He who isacquainted with the distribution of existing species over the globe, willnot attempt to account for the close resemblance of distinct species inclosely consecutive formations, by the physical conditions of the ancientareas having remained nearly the same. Let it be remembered that the formsof life, at least those inhabiting the sea, have changed almostsimultaneously throughout the world, and therefore under the most differentclimates and conditions. Consider the prodigious vicissitudes of climateduring the pleistocene period, which includes the whole glacial epoch, andnote how little the specific forms of the inhabitants of the sea have beenaffected.

On the theory of descent, the full meaning of the fossil remains fromclosely consecutive formations, being closely related, though ranked asdistinct species, is obvious. As the accumulation of each formation hasoften been interrupted, and as long blank intervals have intervened betweensuccessive formations, we ought not to expect to find, as I attempted toshow in the last chapter, in any one or in any two formations, all theintermediate varieties between the species which appeared at thecommencement and close of these periods: but we ought to find afterintervals, very long as measured by years, but only moderately long asmeasured geologically, closely allied forms, or, as they have been calledby some authors, representative species; and these assuredly we do find. We find, in short, such evidence of the slow and scarcely sensiblemutations of specific forms, as we have the right to expect.

ON THE STATE OF DEVELOPMENT OF ANCIENT COMPARED WITH LIVING FORMS.

We have seen in the fourth chapter that the degree of differentiation andspecialisation of the parts in organic beings, when arrived at maturity, isthe best standard, as yet suggested, of their degree of perfection orhighness. We have also seen that, as the specialisation of parts is anadvantage to each being, so natural selection will tend to render theorganisation of each being more specialised and perfect, and in this sensehigher; not but that it may leave many creatures with simple and unimprovedstructures fitted for simple conditions of life, and in some cases willeven degrade or simplify the organisation, yet leaving such degraded beingsbetter fitted for their new walks of life. In another and more generalmanner, new species become superior to their predecessors; for they have tobeat in the struggle for life all the older forms, with which they comeinto close competition. We may therefore conclude that if under a nearlysimilar climate the eocene inhabitants of the world could be put intocompetition with the existing inhabitants, the former would be beaten andexterminated by the latter, as would the secondary by the eocene, and thepalaeozoic by the secondary forms. So that by this fundamental test ofvictory in the battle for life, as well as by the standard of thespecialisation of organs, modern forms ought, on the theory of naturalselection, to stand higher than ancient forms. Is this the case? A largemajority of palaeontologists would answer in the affirmative; and it seemsthat this answer must be admitted as true, though difficult of proof.

It is no valid objection to this conclusion, that certain Brachiopods havebeen but slightly modified from an extremely remote geological epoch; andthat certain land and fresh-water shells have remained nearly the same,from the time when, as far as is known, they first appeared. It is not aninsuperable difficulty that Foraminifera have not, as insisted on by Dr.Carpenter, progressed in organisation since even the Laurentian epoch; forsome organisms would have to remain fitted for simple conditions of life,and what could be better fitted for this end than these lowly organisedProtozoa? Such objections as the above would be fatal to my view, if itincluded advance in organisation as a necessary contingent. They wouldlikewise be fatal, if the above Foraminifera, for instance, could be provedto have first come into existence during the Laurentian epoch, or the aboveBrachiopods during the Cambrian formation; for in this case, there wouldnot have been time sufficient for the development of these organisms up tothe standard which they had then reached. When advanced up to any givenpoint, there is no necessity, on the theory of natural selection, for theirfurther continued process; though they will, during each successive age,have to be slightly modified, so as to hold their places in relation toslight changes in their conditions. The foregoing objections hinge on thequestion whether we really know how old the world is, and at what periodthe various forms of life first appeared; and this may well be disputed.

The problem whether organisation on the whole has advanced is in many waysexcessively intricate. The geological record, at all times imperfect, doesnot extend far enough back to show with unmistakable clearness that withinthe known history of the world organisation has largely advanced. Even atthe present day, looking to members of the same class, naturalists are notunanimous which forms ought to be ranked as highest: thus, some look atthe selaceans or sharks, from their approach in some important points ofstructure to reptiles, as the highest fish; others look at the teleosteansas the highest. The ganoids stand intermediate between the selaceans andteleosteans; the latter at the present day are largely preponderant innumber; but formerly selaceans and ganoids alone existed; and in this case,according to the standard of highness chosen, so will it be said thatfishes have advanced or retrograded in organisation. To attempt to comparemembers of distinct types in the scale of highness seems hopeless; who willdecide whether a cuttle-fish be higher than a bee--that insect which thegreat Von Baer believed to be "in fact more highly organised than a fish,although upon another type?" In the complex struggle for life it is quitecredible that crustaceans, not very high in their own class, might beatcephalopods, the highest molluscs; and such crustaceans, though not highlydeveloped, would stand very high in the scale of invertebrate animals, ifjudged by the most decisive of all trials--the law of battle. Beside theseinherent difficulties in deciding which forms are the most advanced inorganisation, we ought not solely to compare the highest members of a classat any two periods--though undoubtedly this is one and perhaps the mostimportant element in striking a balance--but we ought to compare all themembers, high and low, at two periods. At an ancient epoch the highest andlowest molluscoidal animals, namely, cephalopods and brachiopods, swarmedin numbers; at the present time both groups are greatly reduced, whileothers, intermediate in organisation, have largely increased; consequentlysome naturalists maintain that molluscs were formerly more highly developedthan at present; but a stronger case can be made out on the opposite side,by considering the vast reduction of brachiopods, and the fact that ourexisting cephalopods, though few in number, are more highly organised thantheir ancient representatives. We ought also to compare the relativeproportional numbers, at any two periods, of the high and low classesthroughout the world: if, for instance, at the present day fifty thousandkinds of vertebrate animals exist, and if we knew that at some formerperiod only ten thousand kinds existed, we ought to look at this increasein number in the highest class, which implies a great displacement of lowerforms, as a decided advance in the organisation of the world. We thus seehow hopelessly difficult it is to compare with perfect fairness, under suchextremely complex relations, the standard of organisation of theimperfectly-known faunas of successive periods.

We shall appreciate this difficulty more clearly by looking to certainexisting faunas and floras. From the extraordinary manner in whichEuropean productions have recently spread over New Zealand, and have seizedon places which must have been previously occupied by the indigenes, wemust believe, that if all the animals and plants of Great Britain were setfree in New Zealand, a multitude of British forms would in the course oftime become thoroughly naturalized there, and would exterminate many of thenatives. On the other hand, from the fact that hardly a single inhabitantof the southern hemisphere has become wild in any part of Europe, we maywell doubt whether, if all the productions of New Zealand were set free inGreat Britain, any considerable number would be enabled to seize on placesnow occupied by our native plants and animals. Under this point of view,the productions of Great Britain stand much higher in the scale than thoseof New Zealand. Yet the most skilful naturalist, from an examination ofthe species of the two countries, could not have foreseen this result.

Agassiz and several other highly competent judges insist that ancientanimals resemble to a certain extent the embryos of recent animalsbelonging to the same classes; and that the geological succession ofextinct forms is nearly parallel with the embryological development ofexisting forms. This view accords admirably well with our theory. In afuture chapter I shall attempt to show that the adult differs from itsembryo, owing to variations having supervened at a not early age, andhaving been inherited at a corresponding age. This process, whilst itleaves the embryo almost unaltered, continually adds, in the course ofsuccessive generations, more and more difference to the adult. Thus theembryo comes to be left as a sort of picture, preserved by nature, of theformer and less modified condition of the species. This view may be true,and yet may never be capable of proof. Seeing, for instance, that theoldest known mammals, reptiles, and fishes strictly belong to their properclasses, though some of these old forms are in a slight degree lessdistinct from each other than are the typical members of the same groups atthe present day, it would be vain to look for animals having the commonembryological character of the Vertebrata, until beds rich in fossils arediscovered far beneath the lowest Cambrian strata--a discovery of which thechance is small.

ON THE SUCCESSION OF THE SAME TYPES WITHIN THE SAME AREAS, DURING THE LATERTERTIARY PERIODS.

Mr. Clift many years ago showed that the fossil mammals from the Australiancaves were closely allied to the living marsupials of that continent. InSouth America, a similar relationship is manifest, even to an uneducatedeye, in the gigantic pieces of armour, like those of the armadillo, foundin several parts of La Plata; and Professor Owen has shown in the moststriking manner that most of the fossil mammals, buried there in suchnumbers, are related to South American types. This relationship is evenmore clearly seen in the wonderful collection of fossil bones made by MM.Lund and Clausen in the caves of Brazil. I was so much impressed withthese facts that I strongly insisted, in 1839 and 1845, on this "law of thesuccession of types,"--on "this wonderful relationship in the samecontinent between the dead and the living." Professor Owen hassubsequently extended the same generalisation to the mammals of the OldWorld. We see the same law in this author's restorations of the extinctand gigantic birds of New Zealand. We see it also in the birds of thecaves of Brazil. Mr. Woodward has shown that the same law holds good withsea-shells, but, from the wide distribution of most molluscs, it is notwell displayed by them. Other cases could be added, as the relationbetween the extinct and living land-shells of Madeira; and between theextinct and living brackish water-shells of the Aralo-Caspian Sea.

Now, what does this remarkable law of the succession of the same typeswithin the same areas mean? He would be a bold man who, after comparingthe present climate of Australia and of parts of South America, under thesame latitude, would attempt to account, on the one hand through dissimilarphysical conditions, for the dissimilarity of the inhabitants of these twocontinents; and, on the other hand through similarity of conditions, forthe uniformity of the same types in each continent during the latertertiary periods. Nor can it be pretended that it is an immutable law thatmarsupials should have been chiefly or solely produced in Australia; orthat Edentata and other American types should have been solely produced inSouth America. For we know that Europe in ancient times was peopled bynumerous marsupials; and I have shown in the publications above alluded to,that in America the law of distribution of terrestrial mammals was formerlydifferent from what it now is. North America formerly partook strongly ofthe present character of the southern half of the continent; and thesouthern half was formerly more closely allied, than it is at present, tothe northern half. In a similar manner we know, from Falconer andCautley's discoveries, that Northern India was formerly more closelyrelated in its mammals to Africa than it is at the present time. Analogousfacts could be given in relation to the distribution of marine animals.

On the theory of descent with modification, the great law of the longenduring, but not immutable, succession of the same types within the sameareas, is at once explained; for the inhabitants of each quarter of theworld will obviously tend to leave in that quarter, during the nextsucceeding period of time, closely allied though in some degree modifieddescendants. If the inhabitants of one continent formerly differed greatlyfrom those of another continent, so will their modified descendants stilldiffer in nearly the same manner and degree. But after very long intervalsof time, and after great geographical changes, permitting muchintermigration, the feebler will yield to the more dominant forms, andthere will be nothing immutable in the distribution of organic beings.

It may be asked in ridicule whether I suppose that the megatherium andother allied huge monsters, which formerly lived in South America, haveleft behind them the sloth, armadillo, and anteater, as their degeneratedescendants. This cannot for an instant be admitted. These huge animalshave become wholly extinct, and have left no progeny. But in the caves ofBrazil there are many extinct species which are closely allied in size andin all other characters to the species still living in South America; andsome of these fossils may have been the actual progenitors of the livingspecies. It must not be forgotten that, on our theory, all the species ofthe same genus are the descendants of some one species; so that, if sixgenera, each having eight species, be found in one geological formation,and in a succeeding formation there be six other allied or representativegenera, each with the same number of species, then we may conclude thatgenerally only one species of each of the older genera has left modifieddescendants, which constitute the new genera containing the severalspecies; the other seven species of each old genus having died out and leftno progeny. Or, and this will be a far commoner case, two or three speciesin two or three alone of the six older genera will be the parents of thenew genera: the other species and the other old genera having becomeutterly extinct. In failing orders, with the genera and species decreasingin numbers as is the case with the Edentata of South America, still fewergenera and species will leave modified blood-descendants.

SUMMARY OF THE PRECEDING AND PRESENT CHAPTERS.

I have attempted to show that the geological record is extremely imperfect;that only a small portion of the globe has been geologically explored withcare; that only certain classes of organic beings have been largelypreserved in a fossil state; that the number both of specimens and ofspecies, preserved in our museums, is absolutely as nothing compared withthe number of generations which must have passed away even during a singleformation; that, owing to subsidence being almost necessary for theaccumulation of deposits rich in fossil species of many kinds, and thickenough to outlast future degradation, great intervals of time must haveelapsed between most of our successive formations; that there has probablybeen more extinction during the periods of subsidence, and more variationduring the periods of elevation, and during the latter the record will havebeen least perfectly kept; that each single formation has not beencontinuously deposited; that the duration of each formation is probablyshort compared with the average duration of specific forms; that migrationhas played an important part in the first appearance of new forms in anyone area and formation; that widely ranging species are those which havevaried most frequently, and have oftenest given rise to new species; thatvarieties have at first been local; and lastly, although each species musthave passed through numerous transitional stages, it is probable that theperiods, during which each underwent modification, though many and long asmeasured by years, have been short in comparison with the periods duringwhich each remained in an unchanged condition. These causes, takenconjointly, will to a large extent explain why--though we do find manylinks--we do not find interminable varieties, connecting together allextinct and existing forms by the finest graduated steps. It should alsobe constantly borne in mind that any linking variety between two forms,which might be found, would be ranked, unless the whole chain could beperfectly restored, as a new and distinct species; for it is not pretendedthat we have any sure criterion by which species and varieties can bediscriminated.

He who rejects this view of the imperfection of the geological record, willrightly reject the whole theory. For he may ask in vain where are thenumberless transitional links which must formerly have connected theclosely allied or representative species, found in the successive stages ofthe same great formation? He may disbelieve in the immense intervals oftime which must have elapsed between our consecutive formations; he mayoverlook how important a part migration has played, when the formations ofany one great region, as those of Europe, are considered; he may urge theapparent, but often falsely apparent, sudden coming in of whole groups ofspecies. He may ask where are the remains of those infinitely numerousorganisms which must have existed long before the Cambrian system wasdeposited? We now know that at least one animal did then exist; but I cananswer this last question only by supposing that where our oceans nowextend they have extended for an enormous period, and where our oscillatingcontinents now stand they have stood since the commencement of the Cambriansystem; but that, long before that epoch, the world presented a widelydifferent aspect; and that the older continents, formed of formations olderthan any known to us, exist now only as remnants in a metamorphosedcondition, or lie still buried under the ocean.

Passing from these difficulties, the other great leading facts inpalaeontology agree admirably with the theory of descent with modificationthrough variation and natural selection. We can thus understand how it isthat new species come in slowly and successively; how species of differentclasses do not necessarily change together, or at the same rate, or in thesame degree; yet in the long run that all undergo modification to someextent. The extinction of old forms is the almost inevitable consequenceof the production of new forms. We can understand why, when a species hasonce disappeared, it never reappears. Groups of species increase innumbers slowly, and endure for unequal periods of time; for the process ofmodification is necessarily slow, and depends on many complexcontingencies. The dominant species belonging to large and dominant groupstend to leave many modified descendants, which form new sub-groups andgroups. As these are formed, the species of the less vigorous groups, fromtheir inferiority inherited from a common progenitor, tend to becomeextinct together, and to leave no modified offspring on the face of theearth. But the utter extinction of a whole group of species has sometimesbeen a slow process, from the survival of a few descendants, lingering inprotected and isolated situations. When a group has once whollydisappeared, it does not reappear; for the link of generation has beenbroken.

We can understand how it is that dominant forms which spread widely andyield the greatest number of varieties tend to people the world withallied, but modified, descendants; and these will generally succeed indisplacing the groups which are their inferiors in the struggle forexistence. Hence, after long intervals of time, the productions of theworld appear to have changed simultaneously.

We can understand how it is that all the forms of life, ancient and recent,make together a few grand classes. We can understand, from the continuedtendency to divergence of character, why the more ancient a form is, themore it generally differs from those now living. Why ancient and extinctforms often tend to fill up gaps between existing forms, sometimes blendingtwo groups, previously classed as distinct into one; but more commonlybringing them only a little closer together. The more ancient a form is,the more often it stands in some degree intermediate between groups nowdistinct; for the more ancient a form is, the more nearly it will berelated to, and consequently resemble, the common progenitor of groups,since become widely divergent. Extinct forms are seldom directlyintermediate between existing forms; but are intermediate only by a longand circuitous course through other extinct and different forms. We canclearly see why the organic remains of closely consecutive formations areclosely allied; for they are closely linked together by generation. We canclearly see why the remains of an intermediate formation are intermediatein character.

The inhabitants of the world at each successive period in its history havebeaten their predecessors in the race for life, and are, in so far, higherin the scale, and their structure has generally become more specialised; and this may account for the common belief held by so manypalaeontologists, that organisation on the whole has progressed. Extinctand ancient animals resemble to a certain extent the embryos of the morerecent animals belonging to the same classes, and this wonderful factreceives a simple explanation according to our views. The succession ofthe same types of structure within the same areas during the latergeological periods ceases to be mysterious, and is intelligible on theprinciple of inheritance.

If, then, the geological record be as imperfect as many believe, and it mayat least be asserted that the record cannot be proved to be much moreperfect, the main objections to the theory of natural selection are greatlydiminished or disappear. On the other hand, all the chief laws ofpalaeontology plainly proclaim, as it seems to me, that species have beenproduced by ordinary generation: old forms having been supplanted by newand improved forms of life, the products of variation and the survival ofthe fittest.