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Updated: May 14, 2025
The most novel results comprised in this work were: first, the remodeling of the classification of the whole type of fishes, fossil and living, and especially the separation of the Ganoids from all other fishes, under the rank of a distinct order; second, the recognition of those combinations of reptilian and bird-like characters in the earlier geological fishes, which led the author to call them prophetic types; and third, his discovery of an analogy between the embryological phases of the higher present fishes and the gradual introduction of the whole type on earth, the series in growth and the series in time revealing a certain mutual correspondence.
Thus the human embryo at one stage of its development resembles the fish; at another stage, the embryo of a dog; and for a long time it is impossible to distinguish between the human embryo and that of one of the larger apes. These embryological facts, biologists say, indicate genetic relation between the various animal forms which the embryo in its different stages simulates.
Moreover, the embryonic states of development show also, in all their similarity, even in the very first stages, and with especial distinctness in these first stages, many differences between the single species; and this is true especially of those species which, according to the followers of this so-called biogenetic maxim, should lie in the same stem-line, so that the direct scientific value of the embryological results to the palæontological investigation, or of the latter to the former, is so far very slight.
From some of these "Zoeopoda" arose all the remaining Malacostracous Crustacea; while, from others, was developed some form analogous to the existing Galeodes, out of which proceeded, by gradual differentiation, all the Myriapoda, Arachnida, and Insecta. I should, be disposed to interpret the facts of the embryological history and of the anatomy of the Arthropoda in a different manner.
"the functions of the central nervous system, which were originally taken by the whole skin, became gradually concentrated in a special part of the skin which was step by step removed from the surface, and has finally become in the higher types a well-defined organ imbedded in the subdermal tissues.... The embryological evidence shows that the ganglion-cells of the central part of the nervous system are originally derived from the simple undifferentiated epithelial cells of the surface of the body."
It had long been a favorite idea with him to build up an embryological department in his prospective museum; the more so because such a provision on any large scale had never been included in the plan of the great zoological institutions, and he believed it would have a direct and powerful influence on the progress of modern science.
I believe that no other line of scientific facts is so likely to claim a serious and respectful attitude. The ideal way for giving a popular glimpse at human development is with a small series of lantern slides or photographs from embryological works.
In this study, the poetical intuition of the ancient philosophers is interestingly linked with the experimental spirit and the analytical method of modern science. The latest biological and embryological theories are invoked to help in the comment on the hylozoism of the seven sages and the mysticism of the early Christians. Janicki and de Vries shake hands with Heraclitus and Saint Paul.
We shall hereafter, I think, clearly see why embryological characters are of such high classificatory importance. Geographical distribution may sometimes be brought usefully into play in classing large and widely-distributed genera, because all the species of the same genus, inhabiting any distinct and isolated region, have in all probability descended from the same parents.
You will find some allusion to these results in my Lectures on Embryology, given in my "Lake Superior," of which I have twice sent you a copy, that it might reach you the more surely; but these first impressions have assumed greater coherence now, and I constantly find myself recurring to my fossils for light upon the embryonic forms I am studying and vice versa, consulting my embryological drawings in order to decipher the fossils with greater certainty.
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