Popular Science Monthly/Volume 9/August 1876/What are Species?
|WHAT ARE SPECIES?|
By Prof. T. H. HUXLEY.
IN its most general acceptation the word "species" signifies 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, logically, 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 his plants or his shells out 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 character 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 differentiates them from minerals. This process of generation enters 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 generation the successive zoöids may differ very widely; but, inasmuch as the differing forms in these cases proceed from the same parents, 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, 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 be admitted, and that living beings which arise from totally distinct parents are of different species, even though morphologically identical. The wellnigh exploded hypothesis of the multiplicity of centres of origin for species of wide distribution 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 fauna and flora of successive formations used to affirm that, although indistinguishable, two forms from separate formations must be of distinct species, because they had been created separately. However, these subtilties 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 unfertile 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 many morphological species, yet they, apart from the historical evidence of their parentage, 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 character of the species from which they have proceeded. The conclusion has been drawn that the character of species is physiologically fixed; that is to say, that, however long the process of generation may be continued, the individuals either retain the identical morphological character of the oldest ancestor, or, if they vary, the varieties remain fertile with one another.
Assuming that species have the physiological character thus enumerated, certain conclusions respecting the "origin of species" are inevitable. It is clear that no existing species can have arisen by the intercrossing of preëxisting species, or by the variation of preexisting species, but that every species must 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 formæ sunt creatæ" (we reckon as many species as there were forms created in the beginning) is the well-known formula of Linnæus.
In practice Linnæus regarded species from a purely mythological point of view; in theory, he assumed the common ancestry 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 character of species, which up till within the last few years have been almost universally prevalent.
In the introduction to the "Règne Animal" (1816), 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 constitutes 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 vigorous, 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 successors into curious difficulties. Developing the fundamental idea of the "Discours sur les Révolutions de la Surface du Globe," naturalists were forced to conclude not only that existing species are the result of creation, but that the creative act by which they were brought into being was only the last repetition of a series of such acts by which the often depopulated world has been as frequently repeopled, and thus orthodox belief respecting the existing flora and fauna led to a terribly heterodox cosmogony.
The contemporary and countryman of Cuvier, Lamarck, 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 the "Origin of Species," that it has almost swept opposition away. Lamarck's vast acquaintance with the details of invertebrate zoölogy 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 fact of embryology, the occurrence of rudimentary organs, and the fundamental unity of structure which obtains in vast groups, such as the vertebrata and arthropoda, all tended to suggest the existence of a genetic connection between species, so that Lamarck was finally led 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 modification of a predecessor. Lamarck's views of the nature of geological changes 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 "Discours sur les Révolutions" 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. And 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 utterly vanquished.
Catastrophic geology has vanished out of sight, 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 well established 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 the same as that which should have occurred if the hypothesis of evolution is correct.
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 vertebrata 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 embryologists 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, whether Mount Ararat or any other; and, by demonstrating the similarity of the existing fauna and flora of any locality to that 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, those advocated by Cuvier; and that, even if no suggestion has 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 marshaled 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 differentialism of the living matter into forms having the same 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 tendency 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 the 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, while it is also conceivable that both variation and the direction which variation takes are partly determined by intrinsic and partly by extrinsic conditions.—The American Cyclopœdia.