Systematics

“ It is in this sense that the evidence from comparative anatomy can be used I think as an argument for evolution. It is the resemblances that the animals or plants in any group have in common that is the basis for such a conclusion; it is not because we can arrange in a continuous series any particular variations. In other words, our inference concerning the common descent of two or more species is based on the totality of such resemblances that still remain in large part after each change has taken place. ”

“ In considering this view of classification, it should be borne in mind that the element of descent has been universally used in ranking together the sexes, ages, and acknowledged varieties of the same species, however different they may be in structure. If we extend the use of this element of descent,—the only certainly known cause of similarity in organic beings,—we shall understand what is meant by the natural system: it is genealogical in its attempted arrangement, with the grades of acquired difference marked by the terms varieties, species, genera, families, orders, and classes. ”

“ The history of the origin of a type, as species, genus, order, etc., is simply the history of the origin of the structure or structures which define those groups respectively. It is nothing more nor less than this, whether a man or an insect be the object of investigation. ”

“ You ask how far I go in attributing organisms to a common descent; I answer I know not; the way in which I intend treating the subject, is to show (AS FAR AS I CAN) the facts and arguments for and against the common descent of the species of the same genus; and then show how far the same arguments tell for or against forms, more and more widely different: and when we come to forms of different orders and classes, there remain only some such arguments as those which can perhaps be deduced from similar rudimentary structures, and very soon not an argument is left. ”

“ The theory of evolution finds its broadest inductive foundation in the natural classification of living things, which arranges all the various forms in larger and smaller groups, according to their degree of affinity. These groupings or categories of classification—the varieties, species, genera, families, orders, classes, etc.—show such constant features of coordination and subordination that we are bound to look on them as genealogical, and represent the whole system in the form of a branching tree. ”

“ We have still to admit with Darwin that it is difficult or impossible to assign utility to all the characters that distinguish species, and particularly to those characters by which systematists identify species. The modern tendency for a more complete and detailed separation of individual forms into specific and sub-specific groups, and the immensely larger range of material at the disposal of systematic experts, have combined to make it increasingly difficult to imagine conditions of the environment under which the species of systematists would have been produced by selection. ”

“ 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? ”

“ If we now turn to comparative anatomy, to classification, and to the geographical distribution of animals, we find that it is necessary to assume the doctrine of descent in order to explain the observed facts; the evidence for evolution, indeed, becomes cumulative. But it is not necessary, nor will space permit, to give extended illustrations from these various departments of biological researches. ”

“ Hence modifications of structure, viewed by systematists as of high value, may be wholly due to the laws of variation and correlation, without being, as far as we can judge, of the slightest service to the species. ”

“ Wherefore, if we believe that the species were designed, and that natural propagation was designed, how can we say that the actual varieties of the species were not equally designed? Have we not similar grounds for inferring design in the supposed varieties of species, that we have in the case of the supposed species of a genus? ”

“ Simplicity is the main end of any classification. If all the races, or as they are called, species, of any kingdom were perfectly known, and if the true analogies between each species, and between the groups which species form, were also known, so that their approximations to each other and the position of the several groups were in conformity with the natural analogies between them—then classes, orders, sections, and genera would be families, larger or smaller ”

“ If that aptitude to multiply, to spread, to separate, and to differentiate, which the human races have in all [Pg 252] times shown, be a tendency common to races in general, as we have ample reason to assume, then there will result that kind of relation among the species, and genera, and orders, peopling the earth's surface, which we find exists. Those remarkable identities of type discovered between organisms inhabiting one medium, and strangely-modified organisms inhabiting another medium, are at the same time rendered comprehensible. ”

“ All the foregoing rules and aids and difficulties in classification are explained, if I do not greatly deceive myself, on the view that the natural system is founded on descent with modification; that the characters which naturalists consider as showing true affinity between any two or more species, are those which have been inherited from a common parent, and, in so far, all true classification is genealogical ”

“ Practically, when a naturalist can unite by means of intermediate links any two forms, he treats the one as a variety of the other, ranking the most common, but sometimes the one first described as the species, and the other as the variety. ”

“ The exact nature of the proof of the principle of evolution is that when great masses of scientific data such as are involved in those branches of biology known as taxonomy, comparative anatomy, embryology, serology, paleontology and geographic distribution, are looked upon as the result of evolutionary processes, they take on orderliness, reasonableness, unity and coherency. ”

“ Though The Origin of Species proved to me that the transmission of acquired characters cannot be the sole factor in organic evolution, as I had assumed in Social Statics and in The Principles of Biology, published in pre-Darwinian days, yet I have never wavered in the belief that it is a factor and an all-important factor. ”

“ If we pass up into the kingdoms of the living, and study plants and animals, the same unity of phenomena meets us. Our classification terms—order, genus, species, and so forth—are as artificial as our names for the geological systems. No one holds to-day that any single species is clearly marked off from all others. ”

“ We need not wonder that gradations between species and varieties should occur, or that genera and other groups should not be absolutely limited, though they are represented to be so in our systems. The classifications of the naturalist define abruptly where nature more or less blends. Our systems are nothing if not definite. ”

“ Looking now to the affinities of organisms, without relation to their distribution, and taking all fossil and recent, we see the degrees of relationship are of different degrees and arbitrary,—sub-genera,—genera,—sub-families, families, orders and classes and kingdoms. The kind of classification which everyone feels is most correct is called the natural system, but no can define this. ”

“ Some have made the number of genera and families as few as possible. This results in associating under one name species or genera that present marked differences among themselves. The present tendency of expert opinion is to separate more freely into convenient natural groups, according to similarity of structure, habit, form or appearance. ”

“ Yet it is obvious that we must first settle what are the facts grouped together and indicated by the word “species,” before we can carry on any discussion as to the origin of species. To be battling about the fixity or variability of species, without having rigorously settled what species is, can lead to no edifying result. ”

“ Interpretation of the forms of organisms and the forms of their parts, must depend mainly on the conclusions previously drawn respecting their phylogeny; and the drawing of such conclusions must be guided by recognition of the various factors of Evolution, as well as by recognition of certain extremely general results of Evolution and certain concomitants of Evolution.
A primary one among these is that no existing species can exhibit more than approximately the ancestral structure of any other existing species. ”

“ They are infinitely numerous and often very singular; but are of no use in the classification of the higher groups. How it comes, that certain parts of the structure, by which the habits and functions of the species are settled, are of no use in classification, whilst other parts, formed at the same time, are of the greatest, it would be difficult to say, on the theory of separate creations. ”

“ In all systematic works, which are in any degree thorough, one meets with endless complaints, that here and there species cannot be distinguished because of the excessive number of transition forms. Hence every naturalist defines the limit and the number of individual species differently. Some zoologists and botanists, as I mentioned (vol. i. p. 276) , assume in one and the same group of organisms ten species, others twenty, others a hundred or more, while other systematic naturalists again look upon these different forms only as varieties of a single “good” species. ”

“ 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. ”

“ As species belonging to distinct classes have often been adapted by successive slight modifications to live under nearly similar circumstances,— to inhabit, for instance, the three elements of land, air and water,— we can perhaps understand how it is that a numerical parallelism has sometimes been observed between the subgroups of distinct classes. ”

“ Has each species indeed arisen from one single pair, or have several pairs, resembling each other perfectly both morphologically and physiologically, appeared simultaneously or successively? These are questions of fact which science neither can nor ought to approach, for neither experiment nor observation is able to furnish us with the smallest data requisite for the solution. ”

“ Now in the lower systematic groups this is always the case. Varieties, species, and genera are always distinguished by only relatively small differences; deep-seated distinctions do not here occur, as is implied in the conception of these categories. ”

“ Darwin's doctrine of the origin of species affects the conception of natural classification in several ways, (1) If all living things are blood-relations, modified in the course of ages according to their various conditions of life, 'affinity' must mean 'nearness of common descent'; and it seems irrational to propose a classification upon any other basis. ”

“ We have, in a more general shape, the argument which until lately was thought conclusive—the argument that because the genesis of each species of creature had not been explained, therefore each species must have been separately created. ”

“ It is a fact established without doubt, that the most common and most widely distributed species show the greatest variability, and that those species, on the contrary, which are now rare, although they were, perhaps, at earlier periods, the most common and extremely variable, vary, at the present date, the least of all. ”

“ But all constructive theories of evolution have been built on the understanding that what we know of the relation of varieties to species justifies the assumption that the one phenomenon is a phase of the other, and that each species arises or has arisen from another species either by one or several genetic steps. ”

“ When we confine our attention to any one form, we are deprived of the weighty arguments derived from the nature of the affinities which connect together whole groups of organisms—their geographical distribution in past and present times, and their geological succession. The homological structure, embryological development, and rudimentary organs of a species remain to be considered, whether it be man or any other animal, to which our attention may be directed ”

“ Species is a word, which always has had a [11] double meaning. One is the systematic species, which is the unit of our system. But these units are by no means indivisible. Long ago Linnaeus knew them to be compound in a great number of instances, and increasing knowledge has shown that the same rule prevails in other instances. Today the vast majority of the old systematic species are known to consist of minor units. These minor entities are called varieties in systematic works. ”

“ In organisms, as in all other things, the exposure of different parts to different kinds and amounts of incident forces, has necessitated their differentiation; and, for the like reason, aggregates of individuals have been lapsing into varieties, and species, and genera, and orders. ”

“ This gradual increase in number 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, can increase only slowly and progressively ”

“ Darwin, after making a laborious analysis of many floras, found that those genera which are represented by a large number of species contain a greater number of variable species, relatively speaking, than the smaller genera, or those less numerously represented. ”