Variability -- Individual differences -- Doubtful species -- Wide ranging,much diffused, and common species, vary most -- Species of the largergenera in each country vary more frequently than the species of the smallergenera -- Many of the species of the larger genera resemble varieties inbeing very closely, but unequally, related to each other, and in havingrestricted ranges.

Before applying the principles arrived at in the last chapter to organicbeings in a state of nature, we must briefly discuss whether these latterare subject to any variation. To treat this subject properly, a longcatalogue of dry facts ought to be given; but these I shall reserve for afuture work. Nor shall I here discuss the various definitions which havebeen given of the term species. No one definition has satisfied allnaturalists; yet every naturalist knows vaguely what he means when hespeaks of a species. Generally the term includes the unknown element of adistinct act of creation. The term "variety" is almost equally difficultto define; but here community of descent is almost universally implied,though it can rarely be proved. We have also what are calledmonstrosities; but they graduate into varieties. By a monstrosity Ipresume is meant some considerable deviation of structure, generallyinjurious, or not useful to the species. Some authors use the term"variation" in a technical sense, as implying a modification directly dueto the physical conditions of life; and "variations" in this sense aresupposed not to be inherited; but who can say that the dwarfed condition ofshells in the brackish waters of the Baltic, or dwarfed plants on Alpinesummits, or the thicker fur of an animal from far northwards, would not insome cases be inherited for at least a few generations? And in this case Ipresume that the form would be called a variety.

It may be doubted whether sudden and considerable deviations of structure,such as we occasionally see in our domestic productions, more especiallywith plants, are ever permanently propagated in a state of nature. Almostevery part of every organic being is so beautifully related to its complexconditions of life that it seems as improbable that any part should havebeen suddenly produced perfect, as that a complex machine should have beeninvented by man in a perfect state. Under domestication monstrositiessometimes occur which resemble normal structures in widely differentanimals. Thus pigs have occasionally been born with a sort of proboscis,and if any wild species of the same genus had naturally possessed aproboscis, it might have been argued that this had appeared as amonstrosity; but I have as yet failed to find, after diligent search, casesof monstrosities resembling normal structures in nearly allied forms, andthese alone bear on the question. If monstrous forms of this kind ever doappear in a state of nature and are capable of reproduction (which is notalways the case), as they occur rarely and singly, their preservation woulddepend on unusually favourable circumstances. They would, also, during thefirst and succeeding generations cross with the ordinary form, and thustheir abnormal character would almost inevitably be lost. But I shall haveto return in a future chapter to the preservation and perpetuation ofsingle or occasional variations.

INDIVIDUAL DIFFERENCES.

The many slight differences which appear in the offspring from the sameparents, or which it may be presumed have thus arisen, from being observedin the individuals of the same species inhabiting the same confinedlocality, may be called individual differences. No one supposes that allthe individuals of the same species are cast in the same actual mould. These individual differences are of the highest importance for us, for theyare often inherited, as must be familiar to every one; and they thus affordmaterials for natural selection to act on and accumulate, in the samemanner as man accumulates in any given direction individual differences inhis domesticated productions. These individual differences generallyaffect what naturalists consider unimportant parts; but I could show, by along catalogue of facts, that parts which must be called important, whetherviewed under a physiological or classificatory point of view, sometimesvary in the individuals of the same species. I am convinced that the mostexperienced naturalist would be surprised at the number of the cases ofvariability, even in important parts of structure, which he could collecton good authority, as I have collected, during a course of years. Itshould be remembered that systematists are far from being pleased atfinding variability in important characters, and that there are not manymen who will laboriously examine internal and important organs, and comparethem in many specimens of the same species. It would never have beenexpected that the branching of the main nerves close to the great centralganglion of an insect would have been variable in the same species; itmight have been thought that changes of this nature could have beeneffected only by slow degrees; yet Sir J. Lubbock has shown a degree ofvariability in these main nerves in Coccus, which may almost be compared tothe irregular branching of the stem of a tree. This philosophicalnaturalist, I may add, has also shown that the muscles in the larvae ofcertain insects are far from uniform. Authors sometimes argue in a circlewhen they state that important organs never vary; for these same authorspractically rank those parts as important (as some few naturalists havehonestly confessed) which do not vary; and, under this point of view, noinstance will ever be found of an important part varying; but under anyother point of view many instances assuredly can be given.

There is one point connected with individual differences which is extremelyperplexing: I refer to those genera which have been called "protean" or"polymorphic," in which species present an inordinate amount of variation. With respect to many of these forms, hardly two naturalists agree whetherto rank them as species or as varieties. We may instance Rubus, Rosa, andHieracium among plants, several genera of insects, and of Brachiopodshells. In most polymorphic genera some of the species have fixed anddefinite characters. Genera which are polymorphic in one country seem tobe, with a few exceptions, polymorphic in other countries, and likewise,judging from Brachiopod shells, at former periods of time. These facts arevery perplexing, for they seem to show that this kind of variability isindependent of the conditions of life. I am inclined to suspect that wesee, at least in some of these polymorphic genera, variations which are ofno service or disservice to the species, and which consequently have notbeen seized on and rendered definite by natural selection, as hereafter tobe explained.

Individuals of the same species often present, as is known to every one,great differences of structure, independently of variation, as in the twosexes of various animals, in the two or three castes of sterile females orworkers among insects, and in the immature and larval states of many of thelower animals. There are, also, cases of dimorphism and trimorphism, bothwith animals and plants. Thus, Mr. Wallace, who has lately calledattention to the subject, has shown that the females of certain species ofbutterflies, in the Malayan Archipelago, regularly appear under two or eventhree conspicuously distinct forms, not connected by intermediatevarieties. Fritz Muller has described analogous but more extraordinarycases with the males of certain Brazilian Crustaceans: thus, the male of aTanais regularly occurs under two distinct forms; one of these has strongand differently shaped pincers, and the other has antennae much moreabundantly furnished with smelling-hairs. Although in most of these cases,the two or three forms, both with animals and plants, are not now connectedby intermediate gradations, it is possible that they were once thusconnected. Mr. Wallace, for instance, describes a certain butterfly whichpresents in the same island a great range of varieties connected byintermediate links, and the extreme links of the chain closely resemble thetwo forms of an allied dimorphic species inhabiting another part of theMalay Archipelago. Thus also with ants, the several worker-castes aregenerally quite distinct; but in some cases, as we shall hereafter see, thecastes are connected together by finely graduated varieties. So it is, asI have myself observed, with some dimorphic plants. It certainly at firstappears a highly remarkable fact that the same female butterfly should havethe power of producing at the same time three distinct female forms and amale; and that an hermaphrodite plant should produce from the same seed-capsule three distinct hermaphrodite forms, bearing three different kindsof females and three or even six different kinds of males. Neverthelessthese cases are only exaggerations of the common fact that the femaleproduces offspring of two sexes which sometimes differ from each other in awonderful manner.

DOUBTFUL SPECIES.

The forms which possess in some considerable degree the character ofspecies, but which are so closely similar to other forms, or are so closelylinked to them by intermediate gradations, that naturalists do not like torank them as distinct species, are in several respects the most importantfor us. We have every reason to believe that many of these doubtful andclosely allied forms have permanently retained their characters for a longtime; for as long, as far as we know, as have good and true species. Practically, when a naturalist can unite by means of intermediate links anytwo forms, he treats the one as a variety of the other, ranking the mostcommon, but sometimes the one first described as the species, and the otheras the variety. But cases of great difficulty, which I will not hereenumerate, sometimes arise in deciding whether or not to rank one form as avariety of another, even when they are closely connected by intermediatelinks; nor will the commonly assumed hybrid nature of the intermediateforms always remove the difficulty. In very many cases, however, one formis ranked as a variety of another, not because the intermediate links haveactually been found, but because analogy leads the observer to supposeeither that they do now somewhere exist, or may formerly have existed; andhere a wide door for the entry of doubt and conjecture is opened.

Hence, in determining whether a form should be ranked as a species or avariety, the opinion of naturalists having sound judgment and wideexperience seems the only guide to follow. We must, however, in manycases, decide by a majority of naturalists, for few well-marked andwell-known varieties can be named which have not been ranked as species byat least some competent judges.

That varieties of this doubtful nature are far from uncommon cannot bedisputed. Compare the several floras of Great Britain, of France, or ofthe United States, drawn up by different botanists, and see what asurprising number of forms have been ranked by one botanist as goodspecies, and by another as mere varieties. Mr. H.C. Watson, to whom I lieunder deep obligation for assistance of all kinds, has marked for me 182British plants, which are generally considered as varieties, but which haveall been ranked by botanists as species; and in making this list he hasomitted many trifling varieties, but which nevertheless have been ranked bysome botanists as species, and he has entirely omitted several highlypolymorphic genera. Under genera, including the most polymorphic forms,Mr. Babington gives 251 species, whereas Mr. Bentham gives only 112--adifference of 139 doubtful forms! Among animals which unite for eachbirth, and which are highly locomotive, doubtful forms, ranked by onezoologist as a species and by another as a variety, can rarely be foundwithin the same country, but are common in separated areas. How many ofthe birds and insects in North America and Europe, which differ veryslightly from each other, have been ranked by one eminent naturalist asundoubted species, and by another as varieties, or, as they are oftencalled, geographical races! Mr. Wallace, in several valuable papers on thevarious animals, especially on the Lepidoptera, inhabiting the islands ofthe great Malayan Archipelago, shows that they may be classed under fourheads, namely, as variable forms, as local forms, as geographical races orsub-species, and as true representative species. The first or variableforms vary much within the limits of the same island. The local forms aremoderately constant and distinct in each separate island; but when all fromthe several islands are compared together, the differences are seen to beso slight and graduated that it is impossible to define or describe them,though at the same time the extreme forms are sufficiently distinct. Thegeographical races or sub-species are local forms completely fixed andisolated; but as they do not differ from each other by strongly marked andimportant characters, "There is no possible test but individual opinion todetermine which of them shall be considered as species and which asvarieties." Lastly, representative species fill the same place in thenatural economy of each island as do the local forms and sub-species; butas they are distinguished from each other by a greater amount of differencethan that between the local forms and sub-species, they are almostuniversally ranked by naturalists as true species. Nevertheless, nocertain criterion can possibly be given by which variable forms, localforms, sub species and representative species can be recognised.

Many years ago, when comparing, and seeing others compare, the birds fromthe closely neighbouring islands of the Galapagos Archipelago, one withanother, and with those from the American mainland, I was much struck howentirely vague and arbitrary is the distinction between species andvarieties. On the islets of the little Madeira group there are manyinsects which are characterized as varieties in Mr. Wollaston's admirablework, but which would certainly be ranked as distinct species by manyentomologists. Even Ireland has a few animals, now generally regarded asvarieties, but which have been ranked as species by some zoologists. Several experienced ornithologists consider our British red grouse as onlya strongly marked race of a Norwegian species, whereas the greater numberrank it as an undoubted species peculiar to Great Britain. A wide distancebetween the homes of two doubtful forms leads many naturalists to rank themas distinct species; but what distance, it has been well asked, willsuffice if that between America and Europe is ample, will that betweenEurope and the Azores, or Madeira, or the Canaries, or between the severalislets of these small archipelagos, be sufficient?

Mr. B.D. Walsh, a distinguished entomologist of the United States, hasdescribed what he calls Phytophagic varieties and Phytophagic species. Most vegetable-feeding insects live on one kind of plant or on one group ofplants; some feed indiscriminately on many kinds, but do not in consequencevary. In several cases, however, insects found living on different plants,have been observed by Mr. Walsh to present in their larval or mature state,or in both states, slight, though constant differences in colour, size, orin the nature of their secretions. In some instances the males alone, inother instances, both males and females, have been observed thus to differin a slight degree. When the differences are rather more strongly marked,and when both sexes and all ages are affected, the forms are ranked by allentomologists as good species. But no observer can determine for another,even if he can do so for himself, which of these Phytophagic forms ought tobe called species and which varieties. Mr. Walsh ranks the forms which itmay be supposed would freely intercross, as varieties; and those whichappear to have lost this power, as species. As the differences depend onthe insects having long fed on distinct plants, it cannot be expected thatintermediate links connecting the several forms should now be found. Thenaturalist thus loses his best guide in determining whether to rankdoubtful forms as varieties or species. This likewise necessarily occurswith closely allied organisms, which inhabit distinct continents orislands. When, on the other hand, an animal or plant ranges over the samecontinent, or inhabits many islands in the same archipelago, and presentsdifferent forms in the different areas, there is always a good chance thatintermediate forms will be discovered which will link together the extremestates; and these are then degraded to the rank of varieties.

Some few naturalists maintain that animals never present varieties; butthen these same naturalists rank the slightest difference as of specificvalue; and when the same identical form is met with in two distantcountries, or in two geological formations, they believe that two distinctspecies are hidden under the same dress. The term species thus comes to bea mere useless abstraction, implying and assuming a separate act ofcreation. It is certain that many forms, considered by highly competentjudges to be varieties, resemble species so completely in character thatthey have been thus ranked by other highly competent judges. But todiscuss whether they ought to be called species or varieties, before anydefinition of these terms has been generally accepted, is vainly to beatthe air.

Many of the cases of strongly marked varieties or doubtful species welldeserve consideration; for several interesting lines of argument, fromgeographical distribution, analogical variation, hybridism, etc., have beenbrought to bear in the attempt to determine their rank; but space does nothere permit me to discuss them. Close investigation, in many cases, willno doubt bring naturalists to agree how to rank doubtful forms. Yet itmust be confessed that it is in the best known countries that we find thegreatest number of them. I have been struck with the fact that if anyanimal or plant in a state of nature be highly useful to man, or from anycause closely attracts his attention, varieties of it will almostuniversally be found recorded. These varieties, moreover, will often beranked by some authors as species. Look at the common oak, how closely ithas been studied; yet a German author makes more than a dozen species outof forms, which are almost universally considered by other botanists to bevarieties; and in this country the highest botanical authorities andpractical men can be quoted to show that the sessile and pedunculated oaksare either good and distinct species or mere varieties.

I may here allude to a remarkable memoir lately published by A. deCandolle, on the oaks of the whole world. No one ever had more amplematerials for the discrimination of the species, or could have worked onthem with more zeal and sagacity. He first gives in detail all the manypoints of structure which vary in the several species, and estimatesnumerically the relative frequency of the variations. He specifies above adozen characters which may be found varying even on the same branch,sometimes according to age or development, sometimes without any assignablereason. Such characters are not of course of specific value, but they are,as Asa Gray has remarked in commenting on this memoir, such as generallyenter into specific definitions. De Candolle then goes on to say that hegives the rank of species to the forms that differ by characters nevervarying on the same tree, and never found connected by intermediate states. After this discussion, the result of so much labour, he emphaticallyremarks: "They are mistaken, who repeat that the greater part of ourspecies are clearly limited, and that the doubtful species are in a feebleminority. This seemed to be true, so long as a genus was imperfectlyknown, and its species were founded upon a few specimens, that is to say,were provisional. Just as we come to know them better, intermediate formsflow in, and doubts as to specific limits augment." He also adds that itis the best known species which present the greatest number of spontaneousvarieties and sub-varieties. Thus Quercus robur has twenty-eightvarieties, all of which, excepting six, are clustered round three sub-species, namely Q. pedunculata, sessiliflora and pubescens. The formswhich connect these three sub-species are comparatively rare; and, as AsaGray again remarks, if these connecting forms which are now rare were tobecome totally extinct the three sub-species would hold exactly the samerelation to each other as do the four or five provisionally admittedspecies which closely surround the typical Quercus robur. Finally, DeCandolle admits that out of the 300 species, which will be enumerated inhis Prodromus as belonging to the oak family, at least two-thirds areprovisional species, that is, are not known strictly to fulfil thedefinition above given of a true species. It should be added that DeCandolle no longer believes that species are immutable creations, butconcludes that the derivative theory is the most natural one, "and the mostaccordant with the known facts in palaeontology, geographical botany andzoology, of anatomical structure and classification."

When a young naturalist commences the study of a group of organisms quiteunknown to him he is at first much perplexed in determining whatdifferences to consider as specific and what as varietal; for he knowsnothing of the amount and kind of variation to which the group is subject;and this shows, at least, how very generally there is some variation. Butif he confine his attention to one class within one country he will soonmake up his mind how to rank most of the doubtful forms. His generaltendency will be to make many species, for he will become impressed, justlike the pigeon or poultry fancier before alluded to, with the amount ofdifference in the forms which he is continually studying; and he has littlegeneral knowledge of analogical variation in other groups and in othercountries by which to correct his first impressions. As he extends therange of his observations he will meet with more cases of difficulty; forhe will encounter a greater number of closely-allied forms. But if hisobservations be widely extended he will in the end generally be able tomake up his own mind; but he will succeed in this at the expense ofadmitting much variation, and the truth of this admission will often bedisputed by other naturalists. When he comes to study allied forms broughtfrom countries not now continuous, in which case he cannot hope to findintermediate links, he will be compelled to trust almost entirely toanalogy, and his difficulties will rise to a climax.

Certainly no clear line of demarcation has as yet been drawn betweenspecies and sub-species--that is, the forms which in the opinion of somenaturalists come very near to, but do not quite arrive at, the rank ofspecies; or, again, between sub-species and well-marked varieties, orbetween lesser varieties and individual differences. These differencesblend into each other by an insensible series; and a series impresses themind with the idea of an actual passage.

Hence I look at individual differences, though of small interest to thesystematist, as of the highest importance for us, as being the first steptowards such slight varieties as are barely thought worth recording inworks on natural history. And I look at varieties which are in any degreemore distinct and permanent, as steps toward more strongly marked andpermanent varieties; and at the latter, as leading to sub-species, and thento species. The passage from one stage of difference to another may, inmany cases, be the simple result of the nature of the organism and of thedifferent physical conditions to which it has long been exposed; but withrespect to the more important and adaptive characters, the passage from onestage of difference to another may be safely attributed to the cumulativeaction of natural selection, hereafter to be explained, and to the effectsof the increased use or disuse of parts. A well-marked variety maytherefore be called an incipient species; but whether this belief isjustifiable must be judged by the weight of the various facts andconsiderations to be given throughout this work.

It need not be supposed that all varieties or incipient species attain therank of species. They may become extinct, or they may endure as varietiesfor very long periods, as has been shown to be the case by Mr. Wollastonwith the varieties of certain fossil land-shells in Madeira, and withplants by Gaston de Saporta. If a variety were to flourish so as to exceedin numbers the parent species, it would then rank as the species, and thespecies as the variety; or it might come to supplant and exterminate theparent species; or both might co-exist, and both rank as independentspecies. But we shall hereafter return to this subject.

>From these remarks it will be seen that I look at the term species as onearbitrarily given, for the sake of convenience, to a set of individualsclosely resembling each other, and that it does not essentially differ fromthe term variety, which is given to less distinct and more fluctuatingforms. The term variety, again, in comparison with mere individualdifferences, is also applied arbitrarily, for convenience sake.

WIDE-RANGING, MUCH DIFFUSED, AND COMMON SPECIES VARY MOST.

Guided by theoretical considerations, I thought that some interestingresults might be obtained in regard to the nature and relations of thespecies which vary most, by tabulating all the varieties in severalwell-worked floras. At first this seemed a simple task; but Mr. H.C.Watson, to whom I am much indebted for valuable advice and assistance onthis subject, soon convinced me that there were many difficulties, as didsubsequently Dr. Hooker, even in stronger terms. I shall reserve for afuture work the discussion of these difficulties, and the tables of theproportional numbers of the varying species. Dr. Hooker permits me to addthat after having carefully read my manuscript, and examined the tables, hethinks that the following statements are fairly well established. Thewhole subject, however, treated as it necessarily here is with muchbrevity, is rather perplexing, and allusions cannot be avoided to the"struggle for existence," "divergence of character," and other questions,hereafter to be discussed.

Alphonse de Candolle and others have shown that plants which have very wideranges generally present varieties; and this might have been expected, asthey are exposed to diverse physical conditions, and as they come intocompetition (which, as we shall hereafter see, is a far more importantcircumstance) with different sets of organic beings. But my tables furthershow that, in any limited country, the species which are the most common,that is abound most in individuals, and the species which are most widelydiffused within their own country (and this is a different considerationfrom wide range, and to a certain extent from commonness), oftenest giverise to varieties sufficiently well-marked to have been recorded inbotanical works. Hence it is the most flourishing, or, as they may becalled, the dominant species--those which range widely, are the mostdiffused in their own country, and are the most numerous inindividuals--which oftenest produce well-marked varieties, or, as Iconsider them, incipient species. And this, perhaps, might have beenanticipated; for, as varieties, in order to become in any degree permanent,necessarily have to struggle with the other inhabitants of the country, thespecies which are already dominant will be the most likely to yieldoffspring, which, though in some slight degree modified, still inheritthose advantages that enabled their parents to become dominant over theircompatriots. In these remarks on predominence, it should be understoodthat reference is made only to the forms which come into competition witheach other, and more especially to the members of the same genus or classhaving nearly similar habits of life. With respect to the number ofindividuals or commonness of species, the comparison of course relates onlyto the members of the same group. One of the higher plants may be said tobe dominant if it be more numerous in individuals and more widely diffusedthan the other plants of the same country, which live under nearly the sameconditions. A plant of this kind is not the less dominant because someconferva inhabiting the water or some parasitic fungus is infinitely morenumerous in individuals, and more widely diffused. But if the conferva orparasitic fungus exceeds its allies in the above respects, it will then bedominant within its own class.

SPECIES OF THE LARGER GENERA IN EACH COUNTRY VARY MORE FREQUENTLY THAN THESPECIES OF THE SMALLER GENERA.

If the plants inhabiting a country as described in any Flora, be dividedinto two equal masses, all those in the larger genera (i.e., thoseincluding many species) being placed on one side, and all those in thesmaller genera on the other side, the former will be found to include asomewhat larger number of the very common and much diffused or dominantspecies. This might have been anticipated, for the mere fact of manyspecies of the same genus inhabiting any country, shows that there issomething in the organic or inorganic conditions of that country favourableto the genus; and, consequently, we might have expected to have found inthe larger genera, or those including many species, a larger proportionalnumber of dominant species. But so many causes tend to obscure thisresult, that I am surprised that my tables show even a small majority onthe side of the larger genera. I will here allude to only two causes ofobscurity. Fresh water and salt-loving plants generally have very wideranges and are much diffused, but this seems to be connected with thenature of the stations inhabited by them, and has little or no relation tothe size of the genera to which the species belong. Again, plants low inthe scale of organisation are generally much more widely diffused thanplants higher in the scale; and here again there is no close relation tothe size of the genera. The cause of lowly-organised plants ranging widelywill be discussed in our chapter on Geographical Distribution.

>From looking at species as only strongly marked and well-defined varieties,I was led to anticipate that the species of the larger genera in eachcountry would oftener present varieties, than the species of the smallergenera; for wherever many closely related species (i.e., species of thesame genus) have been formed, many varieties or incipient species ought, asa general rule, to be now forming. Where many large trees grow, we expectto find saplings. Where many species of a genus have been formed throughvariation, circumstances have been favourable for variation; and hence wemight expect that the circumstances would generally still be favourable tovariation. On the other hand, if we look at each species as a special actof creation, there is no apparent reason why more varieties should occur ina group having many species, than in one having few.

To test the truth of this anticipation I have arranged the plants of twelvecountries, and the coleopterous insects of two districts, into two nearlyequal masses, the species of the larger genera on one side, and those ofthe smaller genera on the other side, and it has invariably proved to bethe case that a larger proportion of the species on the side of the largergenera presented varieties, than on the side of the smaller genera. Moreover, the species of the large genera which present any varieties,invariably present a larger average number of varieties than do the speciesof the small genera. Both these results follow when another division ismade, and when all the least genera, with from only one to four species,are altogether excluded from the tables. These facts are of plainsignification on the view that species are only strongly marked andpermanent varieties; for wherever many species of the same genus have beenformed, or where, if we may use the expression, the manufactory of specieshas been active, we ought generally to find the manufactory still inaction, more especially as we have every reason to believe the process ofmanufacturing new species to be a slow one. And this certainly holds trueif varieties be looked at as incipient species; for my tables clearly show,as a general rule, that, wherever many species of a genus have been formed,the species of that genus present a number of varieties, that is, ofincipient species, beyond the average. It is not that all large genera arenow varying much, and are thus increasing in the number of their species,or that no small genera are now varying and increasing; for if this hadbeen so, it would have been fatal to my theory; inasmuch as geology plainlytells us that small genera have in the lapse of time often increasedgreatly in size; and that large genera have often come to their maxima,declined, and disappeared. All that we want to show is, that where manyspecies of a genus have been formed, on an average many are still forming;and this certainly holds good.

MANY OF THE SPECIES INCLUDED WITHIN THE LARGER GENERA RESEMBLE VARIETIES INBEING VERY CLOSELY, BUT UNEQUALLY, RELATED TO EACH OTHER, AND IN HAVINGRESTRICTED RANGES.

There are other relations between the species of large genera and theirrecorded varieties which deserve notice. We have seen that there is noinfallible criterion by which to distinguish species and well-markedvarieties; and when intermediate links have not been found between doubtfulforms, naturalists are compelled to come to a determination by the amountof difference between them, judging by analogy whether or not the amountsuffices to raise one or both to the rank of species. Hence the amount ofdifference is one very important criterion in settling whether two formsshould be ranked as species or varieties. Now Fries has remarked in regardto plants, and Westwood in regard to insects, that in large genera theamount of difference between the species is often exceedingly small. Ihave endeavoured to test this numerically by averages, and, as far as myimperfect results go, they confirm the view. I have also consulted somesagacious and experienced observers, and, after deliberation, they concurin this view. In this respect, therefore, the species of the larger generaresemble varieties, more than do the species of the smaller genera. Or thecase may be put in another way, and it may be said, that in the largergenera, in which a number of varieties or incipient species greater thanthe average are now manufacturing, many of the species already manufacturedstill to a certain extent resemble varieties, for they differ from eachother by a less than the usual amount of difference.

Moreover, the species of the larger genera are related to each other, inthe same manner as the varieties of any one species are related to eachother. No naturalist pretends that all the species of a genus are equallydistinct from each other; they may generally be divided into sub-genera, orsections, or lesser groups. As Fries has well remarked, little groups ofspecies are generally clustered like satellites around other species. Andwhat are varieties but groups of forms, unequally related to each other,and clustered round certain forms--that is, round their parent-species. Undoubtedly there is one most important point of difference betweenvarieties and species, namely, that the amount of difference betweenvarieties, when compared with each other or with their parent-species, ismuch less than that between the species of the same genus. But when wecome to discuss the principle, as I call it, of divergence of character, weshall see how this may be explained, and how the lesser differences betweenvarieties tend to increase into the greater differences between species.

There is one other point which is worth notice. Varieties generally havemuch restricted ranges. This statement is indeed scarcely more than atruism, for if a variety were found to have a wider range than that of itssupposed parent-species, their denominations would be reversed. But thereis reason to believe that the species which are very closely allied toother species, and in so far resemble varieties, often have much restrictedranges. For instance, Mr. H.C. Watson has marked for me in the well-siftedLondon catalogue of Plants (4th edition) sixty-three plants which aretherein ranked as species, but which he considers as so closely allied toother species as to be of doubtful value: these sixty-three reputedspecies range on an average over 6.9 of the provinces into which Mr. Watsonhas divided Great Britain. Now, in this same catalogue, fifty-threeacknowledged varieties are recorded, and these range over 7.7 provinces;whereas, the species to which these varieties belong range over 14.3provinces. So that the acknowledged varieties have very nearly the samerestricted average range, as have the closely allied forms, marked for meby Mr. Watson as doubtful species, but which are almost universally rankedby British botanists as good and true species.

SUMMARY.

Finally, varieties cannot be distinguished from species--except, first, bythe discovery of intermediate linking forms; and, secondly, by a certainindefinite amount of difference between them; for two forms, if differingvery little, are generally ranked as varieties, notwithstanding that theycannot be closely connected; but the amount of difference considerednecessary to give to any two forms the rank of species cannot be defined. In genera having more than the average number of species in any country,the species of these genera have more than the average number of varieties. In large genera the species are apt to be closely but unequally alliedtogether, forming little clusters round other species. Species veryclosely allied to other species apparently have restricted ranges. In allthese respects the species of large genera present a strong analogy withvarieties. And we can clearly understand these analogies, if species onceexisted as varieties, and thus originated; whereas, these analogies areutterly inexplicable if species are independent creations.

We have also seen that it is the most flourishing or dominant species ofthe larger genera within each class which on an average yield the greatestnumber of varieties, and varieties, as we shall hereafter see, tend tobecome converted into new and distinct species. Thus the larger generatend to become larger; and throughout nature the forms of life which arenow dominant tend to become still more dominant by leaving many modifiedand dominant descendants. But, by steps hereafter to be explained, thelarger genera also tend to break up into smaller genera. And thus, theforms of life throughout the universe become divided into groupssubordinate to groups.