Species, in its most general acceptation, a kind or sort of something, which something is the genus to which the species belongs. Thus, a black stone is a species of the genus stone; a gray horse is a species of the genus horse; a scalene triangle is a species of the genus triangle; and, generally, it may be said that every adjective denotes a species of the genus indicated by the substantive to which it is applied. In the technology of the physical sciences the term "species" has a more restricted signification. It is used to denote a group of individuals which corresponds with an early stage of that process of abstraction by which the qualities of individual objects are arranged in the subordinated categories of classification. The individual object alone exists in nature; but, when individual objects are compared, it is found that many agree in all those characters which, for the particular purpose of the classifier, are regarded as important, while they differ only in those which are unimportant; and those which thus agree constitute a species, the definition of which is a statement of the common characters of the individuals which compose the species.
Again, when the species thus established are compared, certain of them are found to agree with one another, and to differ from all the rest in some one or more peculiarities. They thus form a group, which in one sense is merely a species of higher order, while technically it is termed a "genus." And, by a continuation of the same process, genera are grouped into families, families into orders, and so on. Each of the groups thus named is in the logical sense a genus, of which the next lower groups constitute the species. The characters on which species are based necessarily depend upon the nature of the bodies classified. Thus, mineral species are founded upon purely morphological characters; that is to say, they are defined by peculiarities either of form, color, and the like, or of structure; which last term may be used to include both the physical and the chemical characteristics of a mineral. The distinction between a species and a variety is wholly arbitrary, except so far as it is commonly agreed that individuals which differ from others only as terms of a gradual series of modifications belong to the same species, and are to be considered merely as varieties of that species.
It is conceivable that animals and plants should have been known to us only by their remains preserved in museums or in the fossil state. If this had been the case, biological like mineralogical species could have been defined only by morphological characters; that is to say, by the peculiarities of their outward form and inward structure; and, as a matter of fact, this is the state of our knowledge in respect of a large proportion of the existing fauna and flora of the world, and of all extinct animals and plants. A botanist or a conchologist, who sets to work to arrange a newly received collection, sorts out his plants or his shells according to their likenesses and unlikenesses of form and structure, until he has arranged them into groups of individuals which agree in certain constant characters, and differ only by insignificant features, or by such peculiarities as vary in different individuals in such a manner that an insensible gradation can be traced between those forms which have the peculiarity strongly marked and those in which it is absent. Thus far the considerations which guide the biologist in the establishment of species differ in no respect from those which influence the mineralogist.
But although naturalists have no more direct knowledge of any but the morphological characters of the great majority of the species of animals and plants than they would have of so many mineral specimens, they are familiar with many animals and plants in the living state, when they exhibit phenomena to which the mineral world presents no parallel; and the study of these phenomena of active life has complicated the conception of species in biology, by adding physiological to morphological considerations. The fact that living beings originate by generation from other living beings is one of the circumstances in their history which most completely differentiate them from minerals; and ideas derived from the study of the phenomena of generation enter in various ways into the conception of biological species. For example, it is a generally assumed axiom in biology that whatever proceeds from a living being by way of generation is of the same species as that from which it proceeds, whether the morphological differences between parent and offspring be great or small.
The two sexes are often extraordinarily different, and in cases of the so-called "alternation of generations" the successive zooids may differ very widely; but, inasmuch as the differing forms in these cases proceed from one parentage, no one doubts that they belong to the same species. The breeds of domesticated animals and plants often differ morphologically as widely as admitted species do; but, apart from other considerations, historical evidence that they have the same parentage suffices to cause them to be regarded as of one species. It is not quite clear that the converse of the axiom which has just been referred to would now be admitted, and that living beings which arise from totally distinct parents must be held to be of different species, even though morphologically identical. The well-nigh exploded hypothesis of the multiplicity of centres of origin for species of wide distribution, indeed, implies the belief that groups of individuals which have proceeded from distinctly created parents may nevertheless be of the same species; while the supporters of the no less nearly extinct hypothesis of the independent creation of the faunas and floras of successive formations used to affirm that, although indistinguishable, two animals or plants from separate formations must be of distinct species, because they have been created separately.
However, these subtleties have ceased to have any practical importance. In the next place, it is observed that, while individuals of the same morphological species breed freely with one another and give rise to perfectly fertile offspring, the unions of individuals of different morphological species are, as a rule, either infertile or imperfectly fertile. Thus fertility, like parentage, has become a physiological character of species; and though in the case of some domesticated animals, as pigeons, the extreme forms are more different from one another than are many morphological species, yet, apart from the historical evidence of their parentage, they are held to be members of the same species because they are all perfectly fertile one with another, and their offspring are also perfectly fertile. Thirdly, it is a matter of experience that, as a general rule, and taking the whole cycle of forms through which a living being runs into account, offspring and parent are so similar that they belong to one and the same morphological species; and it is further in evidence that many species have endured for extremely long periods without any notable difference being discernible between ancestor and descendant.
Moreover, in some cases, varieties are found to revert to the characters of the species from which they have proceeded. The conclusion has been drawn that species are physiologically fixed; that is to say, that, however long the process of generation maybe continued, the individuals either retain the identical morphological peculiarities of the oldest ancestor, or, if they vary, the varieties remain fertile with one another. Assuming that species have the physiological fixity thus indicated, certain conclusions respecting the origin of species are inevitable. It is clear that no existing species can have arisen by the intercrossing of preexisting species, or by the variation of preexisting species; but that every species must either have existed from all eternity, or have come into existence suddenly in its present form, which is the objective fact denoted by what is termed " creation." - At the dawn of modern biology, a century ago, no scientific evidence respecting the real history of life on the globe was extant, and, for any proof that existed to the contrary, species might have been of eternal duration.
But philosophical speculation combined with theological dogma not only to favor the contrary opinion, but to lead the most philosophic naturalist of his day to embody the hypothesis of creation in a definition of species. Totidem numeramus species quot in principio format sunt creatoe ("We reckon as many species as there were forms created in the beginning"), is the well known formula of Linnaeus. In practice, Linnaeus regarded species from a purely morphological point of view; in theory, he assumed the ancestral creation and the limited variability of species, though he was disposed to allow more freedom in this direction than most of his successors. On the other hand, he seems to have attached comparatively little weight to the assumed sterility of hybrids, and to have held a sort of modified doctrine of evolution, supposing that existing species may have been produced by the interbreeding of comparatively few primordial forms. It is mainly to the influence of Cuvier's authority that we owe the general acceptance of the views respecting the physiological characters of species which till within the last few years have been almost universally prevalent.
In the introduction to the Regne animal (1817), Cuvier writes: "There is no proof that all the differences which now distinguish organized beings are such as may have been produced by circumstances. All that has been advanced upon this subject is hypothetical; experience seems to show, on the contrary, that in the actual state of things varieties are confined within rather narrow limits, and, so far as we can retrace antiquity, we perceive that these limits were the same as at present. We are thus obliged to admit of certain forms which since the origin of things have been perpetuated, without exceeding these limits; and all the beings appertaining to one of these forms constitute what is termed a species. Varieties are accidental subdivisions of species. Generation being the only means of ascertaining the limits to which varieties may extend, species should be defined, the reunion of individuals descended from one another, or from common parents, or from such as resemble them as closely as they resemble each other; but, although this definition is rigorous, it will be seen that its application to particular individuals may be very different when the necessary experiments have been made." It need hardly be said, however, that in practice Cuvier founded his species upon purely and exclusively morphological characters, just as his predecessors and successors have done.
The combination of Cuvier's views on the fixity of species with the discovery of the succession of life on the globe, which was so largely the result of his labors, led his followers into curious difficulties. Developing the fundamental idea of the Discours sur les revolutions de la surface du globe, naturalists were necessarily led to conclude, not only that existing species are the result of creation, but that the creative act which brought them into being was only the last repetition of a series of such acts, by which the often depopulated world has been as frequently repeopled. Lamarck, Cuvier's contemporary and countryman, must be regarded as the chief founder of the reaction against the doctrines which Cuvier advocated; a reaction which, overpowered and disregarded for many years, has acquired such force since and through the publication of Darwin's "Origin of Species," that it has already almost swept opposition away. Lamarck's vast acquaintance with the details of invertebrate zoology rendered him familiar with the great variability of many species, and led him to see that variation is in some way related to change of conditions.
The frequent occurrence of transitional forms between apparently distinct species, when large suites of specimens (especially when they are obtained from different parts of a wide geographical area) are examined, tended to bring into strong light the tenuity of the distinction between species and varieties. The facts of embryology, the occurrence of rudimentary organs, and the fundamental unity of structure which obtains in vast groups, such as the vertebrata and arthropoda, further tended to suggest the existence of a genetic connection between the members of these groups; so that Lamarck was induced to renounce the doctrine of the fixity of species, and to define a species as " a collection of individuals which resemble each other and produce their like by generation, so long as the surrounding conditions do not alter to such an extent as to cause their habits, characters, and forms to vary." According to this definition, the distinction between species and variety once more becomes conventional. A variety is, in fact, a nascent species; and the notion of the creation of species vanishes, inasmuch as every species is the result of the moditication of a predecessor.
Lamarck's views of the nature of geological change were in harmony with his biological speculations, and wholesale catastrophic revolutions were as completely excluded from the one as from the other. It is impossible to read the Dis-cours sur les revolutions of Cuvier and the Principes of Lamarck without being struck with the superiority of the former in sobriety of thought, precision of statement, and coolness of judgment. But it is no less impossible to consider the present state of biological science without being impressed by the circumstance that it is the conception of Lamarck which has triumphed, and that of Cuvier which has been vanquished. Catastrophic geology has vanished, and is everywhere replaced by the conception of slow and gradual change. With it has disappeared the once prevalent notion that the whole living population of the earth has been swept away and replaced in successive epochs. On the contrary, it is now certain that the changes which have taken place in that population have been effected by the slow and gradual substitution of species for species. Moreover, it is well established that, in some cases, the succession of forms in time is just such as that which should have occurred if the hypothesis of evolution is well founded.
The rapid advance of comparative anatomy has diminished or removed the wide intervals which formerly appeared to separate the different divisions of the animal and vegetable kingdoms from one another. Even the hiatus between the verte-brata and the invertebrata is bridged over by recent discovery. The establishment of the cell theory, however much the views originally propounded by Schwann have been modified, leaves no doubt that there is a fundamental similarity in minute structure not only between all animals, but between them and plants; while the discoveries of embrvologists have proved that even the most complex forms of living beings do, in the course of their development, run through a series of changes of the same order as those which are postulated by the evolution theory for life in time. Again, the facts of geographical distribution, as now known, are absolutely incompatible with the hypothesis that existing animals and plants have migrated from a common centre, and, by demonstrating the similarity of the existing fauna and flora of any locality to those which inhabited the same area in the immediately precedent epoch, have furnished a strong argument in favor of the modifiability of species.
Thus, it is not too much to say that the facts of biology known at the present day are all consistent with and in favor of the view of species entertained by Lamarck, while they are unfavorable to, if not incompatible with, that advocated by Cuvier; and that, even if no suggestion had been offered, or could be offered, as to the causes which have led to the gradual evolution of species, the hypothesis that they have arisen by such a process of evolution would be the only one which would have any scientific foundation. - The great service which has been rendered to science by Mr. Darwin, in the "Origin of Species," is that, in the first place, he has marshalled the ascertained facts of biology in such a manner as to render this conclusion irresistible; and secondly, that he has proved the following proposition : Given the existence of living matter endowed with variability, the interaction of. variation with the conditions of existence must tend to give rise to a differentiation of that living matter into forms having such morphological relations as are exhibited by the varieties and species which actually exist in nature.
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; and secondly, the elucidation of the nature of variability. It is conceivable that both the ten-dencv to vary and the directions in which that tendency takes effect are determined by the molecular constitution of a living body; in which case, the operation of changes of external conditions will be indirect, and, so to speak, permissive. It is conceivable, on the other hand, that the tendency to vary is both originated and directed by the influence of external conditions; or that both variation and the direction which variation takes are partly determined by intrinsic and partly by extrinsic conditions. In this case, surrounding circumstances must be regarded as, to a greater or less extent, the true causes of variation.