But whatever its defects, the classification of Linnæus was the first attempt at grouping animals together according to certain common structural characters. His followers and pupils engaged at once in a scrutiny of the differences and similarities among animals, which soon led to a great increase in the number of classes: instead of six, there were presently nine, twelve, and more. But till Cuviers time there was no great principle of classification. Facts were accumulated and more or less systematized, but they were not yet arranged according to law; the principle was still wanting by which to generalize them and give meaning and vitality to the whole. It was Cuvier who found the key. He himself tells us how he first began, in his investigations upon the internal organization of animals, to use his dissections with reference to finding the true relations between animals, and how, ever after, his knowledge of anatomy assisted him in his classifications, and his classifications threw new light again on his anatomical investigations,each science thus helping to fertilize the other. He was not one of those superficial observers who are in haste to announce every new fact that they chance to find, and his first paper2 specially devoted to classification gave to the world the ripe fruit of years of study. This was followed by his great work, Le Règne Animal. He said that animals were united in their most comprehensive groups, not on special characters, but on different plans of structure,moulds, he called them, in which all animals had been cast. He tells us this in such admirable language that I must, to do justice to his thought, give it in his own words:
Si lon considère le règne animal daprès les principes que nous venons de poser en se débarrassant des préjugés établis sur les divisions anciennement admises, en nayant égard quà lorganisation et à la nature des animaux, et non pas à leur grandeur, à leur utilité, au plus ou moins de connaissance que nous en avons, ni à toutes les autres circonstances accessoires, on trouvera quil existe quatre formes principales, quatre plans généraux, si lon peut sexprimer ainsi, daprès lesquels tous les animaux semblent avoir été modelés, et dont les divisions ultérieures, de quelque titre que les naturalistes les aient décorées, ne sont que des modifications assez légères, fondées sur le développement ou laddition de quelques parties, qui ne changent rien à lessence du plan.
The value of this principle was soon tested by its application to facts already known, and it was found that animals whose affinities had been questionable before were now at once referred to their true relations with other animals by ascertaining whether they were built on one or another of these plans. Of such plans or structural conceptions Cuvier found in the whole animal kingdom only four, which he called Vertebrates, Mollusks, Articulates, and Radiates.
With this new principle as the basis of investigation, it was no longer enough for the naturalist to know a certain amount of features characteristic of a certain number of animals,he must penetrate deep enough into their organization to find the secret of their internal structure. Till he can do this, he is like the traveller in a strange city, who looks on the exterior of edifices entirely new to him, but knows nothing of the plan of their internal architecture. To be able to read in the finished structure the plan on which the whole is built is now essential to every naturalist.
There have been many criticisms on this division of Cuviers, and many attempts to change it; but though some improvements have been made in the details of his classification, all departures from its great fundamental principle are errors, and do but lead us away from the recognition of the true affinities among animals.
Each of these plans may be stated in the most general terms. In the Vertebrates there is a vertebral column terminating in a prominent head; this column has an arch above and an arch below, forming a double internal cavity. The parts are symmetrically arranged on either side of the longitudinal axis of the body. In the Mollusks, also, the parts are arranged according to a bilateral symmetry on either side of the body, but the body has but one cavity, and is a soft, concentrated mass, without a distinct individualization of parts. In the Articulates there is but one cavity, and the parts are here again arranged on either side of the longitudinal axis, but in these animals the whole body is divided from end to end into transverse rings or joints movable upon each other. In the Radiates we lose sight of the bilateral symmetry so prevalent in the other three, except as a very subordinate element of structure; the plan of this lowest type is an organic sphere, in which all parts bear definite relations to a vertical axis.
It is not upon any special features, then, that these largest divisions of the animal kingdom are based, but simply upon the general structural idea. Striking as this statement was, it was coldly received at first by contemporary naturalists: they could hardly grasp Cuviers wide generalizations, and perhaps there was also some jealousy of the grandeur of his views. Whatever the cause, his principle of classification was not fully appreciated; but it opened a new road for study, and gave us the keynote to the natural affinities among animals. Lamarck, his contemporary, not recognizing the truth of this principle, distributed the animal kingdom into two great divisions, which he calls Vertebrates and Invertebrates. Ehrenberg also, at a later period, announced another division under two heads,those with a continuous solid nervous centre, and those with merely scattered nervous swellings.3
But there was no real progress in either of these latter classifications, so far as the primary divisions are concerned; for they correspond to the old division of Aristotle, under the head of animals with or without blood, the Enaima and Anaima. This coincidence between systems based on different foundations may teach us that every structural combination includes certain inherent necessities which will bring animals together on whatever set of features we try to classify them; so that the division of Aristotle, founded on the circulating fluids, or that of Lamarck, on the absence or presence of a backbone, or that of Ehrenberg, on the differences of the nervous system, cover the same ground. Lamarck attempted also to use the faculties of animals as a groundwork for division among them. But our knowledge of the psychology of animals is still too imperfect to justify any such use of it. His divisions into Apathetic, Sensitive, and Intelligent animals are entirely theoretical. He places, for instance, Fishes and Reptiles among the Intelligent animals, as distinguished from Crustacea and Insects, which he refers to the second division. But one would be puzzled to say how the former manifest more intelligence than the latter, or why the latter should be placed among the Sensitive animals. Again, some of the animals that he calls Apathetic have been proved by later investigators to show an affection and care for their young, seemingly quite inconsistent with the epithet he has applied to them. In fact, we know so little of the faculties of animals that any classification based upon our present information about them must be very imperfect.
Many modifications of Cuviers great divisions have been attempted. Some naturalists, for instance, have divided off a part of the Radiates and Articulates, insisting upon some special features of structure, and mistaking these for the more important and general characteristics of their respective plans. All subsequent investigations of such would-be improvements show them to be retrograde movements, only proving more clearly that Cuvier detected in his four plans all the great structural ideas on which the vast variety of animals is founded. This result is of greater importance than may at first appear. Upon it depends the question, whether all such classifications represent merely individual impressions and opinions of men, or whether there is really something in Nature that presses upon us certain divisions among animals, certain affinities, certain limitations, founded upon essential principles of organization. Are our systems the inventions of naturalists, or only their reading of the Book of Nature? and can that book have more than one reading? If these classifications are not mere inventions, if they are not an attempt to classify for our own convenience the objects we study, then they are thoughts which, whether we detect them or not, are expressed in Nature,then Nature is the work of thought, the production of intelligence carried out according to plan, therefore premeditated,and in our study of natural objects we are approaching the thoughts of the Creator, reading His conceptions, interpreting a system that is His and not ours.