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But as it has become the fashion to lay the chief stress in the whole question of man's origin on the "descent from the apes," I am compelled to return to it once more, and recall the facts of comparative anatomy and ontogeny that give a decisive answer to this "ape-question."
All the interesting phenomena that we meet in ontogeny and paleontology, comparative anatomy and dysteleology, the distribution and habits of organisms all the important general laws that we abstract from the phenomena of these sciences, and combine in harmonious unity are the broad bases of our great biological induction.
The more we go into the details of organic development, and the more closely we follow the rise of the various parts, the more we see the inseparable connection of embryology and stem-history. The ontogeny of the organs can only be understood in the light of their phylogeny, just as we found of the embryology of the whole body. Each embryonic form is determined by a corresponding stem-form.
But when we recognise the causal correlation of ontogeny and phylogeny expressed in this law, the wonderful facts of embryology are susceptible of a very simple explanation; they are found to be the necessary mechanical effects of the evolution of the stem, determined by the laws of heredity and adaptation.
A similar argument is used in regard to “ontogeny recapitulating phylogeny.” Palæontology does not disclose in the plant-world any “synthetic types,” which might have been the common primitive stock from which many now divergent branches have sprung, nor does it disclose any “transition links” really intermediate, for instance, between cryptogams and gymnosperms, or between gymnosperms and angiosperms.
Little by little I became familiar with the principles of embryonic evolution, ontogeny, and of biological evolution, phylogeny; realized, for the first time, my own history and that of the ancestors from whom I had developed and descended. I, this marvellously complicated being, torn by desires and despairs, was the result of the union of two microscopic cells.
Even Agassiz, who was a believer in special creation and an opponent of evolution, was constrained to point out many other instances, mainly among the invertebrata, where there was a like correspondence between the ontogeny of existing species and their phylogenetic history as revealed by the fossil remains of their ancestors.
The origin of the Mullerian duct is still obscure; comparative anatomy and ontogeny seem to indicate that it originates by differentiation from the Wolffian duct. Both open behind into the cloaca. However uncertain the origin of the nephroduct and its two products, the Mullerian and the Wolffian ducts, may be, its later development is clear enough.
It can only be said positively that the peculiar ontogeny of the complicated optic apparatus in man follows just the same laws as in all the other Vertebrates. Their eye is a part of the fore brain, which has grown forward towards the skin, not an original cutaneous sense-organ, as in the Invertebrates.
Here again, therefore, on the ground of the pithecometra-principle, comparative anatomy and ontogeny teach with full confidence the descent of man from the ape. Now that we have traversed the wonderful region of human embryology and are familiar with the principal parts of it, it will be well to look back on the way we have come, and forward to the further path to truth to which it has led us.
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