Speciation

“ On the ordinary view of each species having been independently created, why should the specific characters, or those by which the species of the same genus differ from each other, be more variable than the generic characters in which they all agree? Why, for instance, should the colour of a flower be more likely to vary in any one species of a genus, if the other species, supposed to have been created independently, have differently coloured flowers, than if all the species of the genus have the same coloured flowers? ”

“ In our second and fourth chapters, on Variation and on Natural Selection, I have attempted to show that it is the widely ranging, the much diffused and common, that is the dominant species belonging to the larger genera, which vary most. The varieties, or incipient species, thus produced ultimately become converted, as I believe, into new and distinct species; and these, on the principle of inheritance, tend to produce other new and dominant species. Consequently the groups which are now large, and which generally include many dominant species, tend to go on increasing indefinitely in size. ”

“ Is it not probable that the varieties which spring up within the limits of particular species are further adaptations of structure to the circumstances under which the tribe is destined to exist? Varieties branch out from the common form of a species, just as the forms of species deviate from the common type of a genus. Why should the one class of phenomena be without end or utility, a mere effect of contingency or chance, more than the other? ”

“ In the first place, varieties, even strongly-marked ones, though having somewhat of the character of species — as is shown by the hopeless doubts in many cases how to rank them — yet certainly differ far less from each other than do good and distinct species. Nevertheless according to my view, varieties are species in the process of formation, or are, as I have called them, incipient species. How, then, does the lesser difference between varieties become augmented into the greater difference between species? ”

“ In another work I shall discuss, if time and health permit, the variability of organic beings in a state of nature; namely, the individual differences presented by animals and plants, and those slightly greater and generally inherited differences which are ranked by naturalists as varieties or geographical races. We shall see how difficult, or rather how impossible it often is, to distinguish between races and sub-species, as the less well- marked forms have sometimes been denominated; and again between sub-species and true species. ”

“ In accordance with our present conception there is selection of species and selection within the species. The struggle for life determines which of a group of elementary species shall survive and which shall disappear. In agricultural practice the corresponding process is usually designated by the name of variety-testing. Within the species, or within the variety, the sieve of natural selection is constantly eliminating poor specimens and preserving those that are best adapted to live under the given conditions. ”

“ Observation further acquaints us with the fact, that, sometimes, an individual member of a species may exhibit a more or less marked variation, which is propagated through all the offspring of that individual, and may even become intensified in them. And, in this manner, a variety, or race, is generated within the species; which variety, or race, if nothing were known respecting its origin, might have every claim to be regarded as a separate morphological species. ”

“ III. The only natural classification of the vast aggregate of diversified forms which this group contains, will be one which ranges them according to their mode and degree of divergence from a small number of principal family types; and any subordinate groupings of genera and species which may be adopted for the convenience of description and nomenclature, must be regarded merely as assemblages characterized by the nature and degree of the modifications of the original type which they may have respectively acquired in the course of genetic descent from a common ancestry. ”

“ Under the head of Variation, in the Family Balanidæ and under the Genus Balanus, I have enlarged on this subject, and have shown that there is scarcely a single external character which is not highly variable in most of the species. As whole groups of specimens often vary in exactly the same manner, it is not easy to exaggerate the difficulty of discriminating species and varieties. ”

“ We may, then, allow for all individual variation while defining species as populations with common heredity. The older definition of a species as a group of similar (implying nearly identical) individuals fails because of the amount of variability actually found in nature. Definitions of species as groups of individuals distinguished by a definite number of diagnostic characters, or by certain degrees of difference from other species lead to artificial concepts that take no adequate account of individual variation, Mendelian inheritance, hybridization, or mutation. ”

“ It seems obvious that the theory of the origin of species [281] by natural selection necessarily involves the divergence of the forms selected. An individual which varies, ipso facto diverges from the type of its species; and its progeny, in which the variation becomes intensified by selection, must diverge still more, not only from the parent stock, but from any other race of that stock starting from a variation of a different character. ”

“ With the exception of fertility, domestic varieties resemble species when crossed in transmitting their characters in the same unequal manner to their offspring, in being subject to the prepotency of one form over the other, and in their liability to reversion. By repeated crosses a variety or a species may be made completely to absorb another. Varieties, as we shall see when we treat of their antiquity, sometimes inherit their new characters almost, or even quite, as firmly as species. ”

“ As showing how diversified are the means of occasional transport, I have discussed at some little length the means of dispersal of fresh-water productions.
If the difficulties be not insuperable in admitting that in the long course of time all the individuals of the same species, and likewise of the several species belonging to the same genus, have proceeded from some one source; then all the grand leading facts of geographical distribution are explicable on the theory of migration, together with subsequent modification and the multiplication of new forms. ”

“ From these considerations, I shall devote the first chapter of this abstract to variation under domestication. We shall thus see that a large amount of hereditary modification is at least possible; and, what is equally or more important, we shall see how great is the power of man in accumulating by his selection successive slight variations. I will then pass on to the variability of species in a state of nature ”

“ In ordinary cases nature has not allowed her race to {11} be contaminated with a cross of another race, and agriculturists know how difficult they find always to prevent this,—effect would be trueness. This character and sterility when crossed, and generally a greater amount of difference, are two main features, which distinguish domestic races from species. ”

“ What is needed for the completion of the theory of the origin of species is, first, definite proof that selective breeding is competent to convert permanent races into physiologically distinct species ”

“ Causes of the difficulty of discriminating species.—But, in point of fact, our new acquisitions consist, more and more as we advance, of specimens brought from foreign and often very distant and barbarous countries. A large proportion have never even been seen alive by scientific inquirers. Instead of having specimens of the young, the adult, and the aged individuals of each sex, and possessing means of investigating the anatomical structure, the peculiar habits, and instincts of each, what is usually the state of our information? ”

“ After this glance at the variations peculiar to each species, a more important consideration presents itself, that of the change of the species themselves; that ancient and immemorial degeneration made in each family, or in every genus, under which we may comprehend the proximating species. Among all terrestrial animals there are only a few detached species, which, like the human, at once compose both species and genus. ”

“ Specific differences divide genus into species.According then, to the differences which produce another thing do the divisions of genera into species arise, and the definitions arising from genus and such differences are assigned. On the other hand, as to those which only make a thing different in quality, diversities alone consist, and the changes of subsistence of a thing ”

“ Varieties are the latest offshoots of the genealogical tree in "an unlineal" order; species, those of an earlier date, but of no definite distinction; genera, more ancient species, and so on. The human races, upon this view, likewise may or may not be species according to the notions of each naturalist as to what differences are specific; but, if not species already, those races that last long enough are sure to become so. It is only a question of time. ”

“ But he who will take the trouble to reflect on the steps by which this first degree of sterility could be increased through natural selection to that high degree which is common with so many species, and which is universal with species which have been differentiated to a generic or family rank, will find the subject extraordinarily complex. ”

“ That this does habitually happen, we must infer from most of the innumerable species throughout nature presenting well-marked differences; whereas varieties, the supposed prototypes and parents of future well-marked species, present slight and ill-defined differences. Mere chance, as we may call it, might cause one variety to differ in some character from its parents, and the offspring of this variety again to differ from its parent in the very same character and in a greater degree ”

“ I see no serious objections to the formation of varieties by natural selection in the existing world, and that, so far as earlier epochs are concerned, this law may be assumed to explain the origin of closely allied species, supposing for this purpose a very long period of time. ”

“ When we look to the hereditary varieties or races of our domestic animals and plants, and compare them with closely allied species, we generally perceive in each domestic race, as already remarked, less uniformity of character than in true species. Domestic races of the same species, also, often have a somewhat monstrous character ”

“ Witness the human races, for instance. Wild species also vary, perhaps about as widely as those of domestication, though in different ways. Some of them apparently vary little, others moderately, others immoderately, to the great bewilderment of systematic botanists and zoologists, and increasing disagreement as to whether various forms shall be held to be original species or strong varieties. Moreover, the degree to which the descendants of the same stock, varying in different directions, may at length diverge, is unknown. ”

“ We see here illustrated in all but the final stage that process by which a solitary widely-ranging species, alone representing its genus, becomes ultimately in each group the parent of a number of peculiar species. The polymorphous, or extremely variable, species plays in this period the all-important part. ”

“ In discussing this subject we shall be enabled at the same time to consider a point equally important for us, namely, whether the several species of a genus which must on our theory all be descended from a common progenitor, can have migrated, undergoing modification during their migration from some one area. ”

“ Natural conditions do not permit of the existence of too many species or sub-species. But it is clear that there are types, call them genera, species, or what you will, that have, by virtue of some inherent fitness and flexibility of adaptation, survived and mastered other types. ”

“ Hence, in determining whether a form should be ranked as a species or a variety, the opinion of naturalists having sound judgment and wide experience seems the only guide to follow. We must, however, in many cases, decide by a majority of naturalists, for few well-marked and well-known varieties can be named which have not been ranked as species by at least some competent judges. ”

“ By the theory of natural selection all living species have been connected with the parent-species of each genus, by differences not greater than we see between the natural and domestic varieties of the same species at the present day ”

“ Contemporary discussion is almost entirely centred upon species. Little is said about genera, and still less of the higher divisions. We do not, in any case, find what we require: the determination of constitutive elements, of general acceptance, which shall be for the genus, family, order, or class, the equivalent of the two denotative marks—morphological and physiological—that are attributed to species. ”

“ Therefore we must try to show that elementary species are distinguished from each other by the acquisition of new qualities, and that varieties are derived from their species either by the reduction of one or more characteristics to the latent state, or by the energizing of dormant characters. ”