[3] For more details upon the different systems of Zooelogy, see
        Agassiz’s Essay on Classification in his Contributions to the
        Natural History of the United States, Vol.  I.

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 Cuvier’s 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