Whoever looks at the immensity of the animal kingdom, and observes how intelligibly and systematically it is arranged in these various divisions, will admit that, however imperfect, the scheme is a magnificent product of human ingenuity and labour. It is not an arbitrary arrangement, like the grouping of the stars in constellations; it expresses, though obscurely, the real order of Nature. All true Classification should be to forms what laws are to phenomena: the one reducing varieties to systematic order, as the other reduces phenomena to their relation of sequence. Now if it be true that the classification expresses the real order of nature, and not simply the order which we may find convenient, there will be something more than mere resemblance indicated in the various groups; or, rather let me say, this resemblance itself is the consequence of some community in the things compared, and will therefore be the mark of some deeper cause. What is this cause? Mr. Darwin holds that “propinquity of descent—the only known cause of the similarity of organic beings—is the bond, hidden as it is by various degrees of modification, which is partially revealed to us by our classifications”[10]—“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; that community of descent is the hidden bond which naturalists have been unconsciously seeking, and not some unknown plan of creation, or the enunciation of general propositions, and the mere putting together and separating objects more or less alike.”[11]
Before proceeding to open the philosophical discussion which inevitably arises on the mention of Mr. Darwin’s book, I will here set down the chief groups, according to Cuvier’s classification, for the benefit of the tyro in natural history, who will easily remember them, and will find the knowledge constantly invoked.
There are four Sub-kingdoms, or Branches:—1. Vertebrata. 2. Mollusca. 3. Articulata. 4. Radiata.
The Vertebrata consist of four classes:—Mammalia, Birds, Reptiles, and Fishes.
The Mollusca consist of six classes:—Cephalopoda (cuttlefish), Pteropoda, Gasteropoda (snails, &c.), Acephala (oysters, &c.), Brachiopoda, and Cirrhopoda (barnacles).—N.B. This last class is now removed from the Molluscs and placed among the Crustaceans.
The Articulata are composed of four classes:—Annelids (worms), Crustacea (lobsters, crabs, &c.), Arachnida (spiders), and Insecta.
The Radiata embrace all the remaining forms; but this group has been so altered since Cuvier’s time, that I will not burden your memory just now with an enumeration of the details.
The reader is now in a condition to appreciate the general line of argument adopted in the discussion of Mr. Darwin’s book, which is at present exciting very great attention, and which will, at any rate, aid in general culture by opening to many minds new tracts of thought. The benefit in this direction is, however, considerably lessened by the extreme vagueness which is commonly attached to the word “species,” as well as by the great want of philosophic culture which impoverishes the majority of our naturalists. I have heard, or read, few arguments on this subject which have not impressed me with the sense that the disputants really attached no distinct ideas to many of the phrases they were uttering. 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.
It is notorious that if you ask even a zoologist, What is a species? you will almost always find that he has only a very vague answer to give; and if his answer be precise, it will be the precision of error, and will vanish into contradictions directly it is examined. The consequence of this is, that even the ablest zoologists are constantly at variance as to specific characters, and often cannot agree whether an animal shall be considered of a new species, or only a variety. There could be no such disagreements if specific characters were definite: if we knew what species meant, once and for all. Ask a chemist, What is a salt? What an acid? and his reply will be definite, and uniformly the same: what he says, all chemists will repeat. Not so the zoologist. Sometimes he will class two animals as of different species, when they only differ in colour, in size, or in the numbers of tentacles, &c.; at other times he will class animals as belonging to the same species, although they differ in size, colour, shape, instincts, habits, &c. The dog, for example, is said to be one species with many varieties, or races. But contrast the pug-dog with the greyhound, the spaniel with the mastiff, the bulldog with the Newfoundland, the setter with the terrier, the sheepdog with the pointer: note the striking differences in their structure and their instincts: and you will find that they differ as widely as some genera, and as most species. If these varieties inhabited different countries—if the pug were peculiar to Australia, and the mastiff to Spain—there is not a naturalist but would class them as of different species. The same remark applies to pigeons and ducks, oxen and sheep.
The reason of this uncertainty is that the thing Species does not exist: the term expresses an abstraction, like Virtue, or Whiteness; not a definite concrete reality, which can be separated from other things, and always be found the same. Nature produces individuals; these individuals resemble each other in varying degrees; according to their resemblances we group them together as classes, orders, genera, and species; but these terms only express the relations of resemblance, they do not indicate the existence of such things as classes, orders, genera, or species.[12] There is a reality indicated by each term—that is to say, a real relation; but there is no objective existence of which we could say, This is variable, This is immutable. Precisely as there is a real relation indicated by the term Goodness, but there is no Goodness apart from the virtuous actions and feelings which we group together under this term. It is true that metaphysicians in past ages angrily debated respecting the Immutability of Virtue, and had no more suspicion of their absurdity, than moderns have who debate respecting the Fixity of Species. Yet no sooner do we understand that Species means a relation of resemblance between animals, than the question of the Fixity, or Variability, of Species resolves itself into this: Can there be any variation in the resemblances of closely allied animals? A question which would never be asked.