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Updated: May 21, 2025
The phylogeny of the placenta has become more intelligible from the fact that we have found a number of transitional forms of it. The four great legions of the Placentals Rodents, Ungulates, Carnassia, and Primates are sharply separated to-day by important features of organisation.
The following five lectures contain the most original matter of any, being devoted to "Phylogeny," or the working out of the details of the process of Evolution in the animal and vegetable kingdoms, so as to prove the line of descent of each group of living beings, and to furnish it with its proper genealogical tree, or "phylum."
It is the most important source of all for special phylogeny; but it has great defects, and often fails us. We must, above all, clearly distinguish between palingenetic and cenogenetic phenomena. We must never forget that the laws of curtailed and disturbed heredity often make the original course of development almost unrecognisable.
But, for the present, in view of this patent incompleteness of our chief sources of evidence, we must naturally be careful not to lay too much stress in human phylogeny on the particular animals we have studied, or regard all the various stages of development with equal confidence as stem-forms.
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.
The ontogenetic loss of the gills and the tail in the frog and toad can only be explained on the assumption that they are descended from long-tailed Amphibia of the salamander type. This is also clear from the comparative anatomy of the two groups. This remarkable metamorphosis is, however, also interesting because it throws a certain light on the phylogeny of the tail-less apes and man.
A pentacrinoid Echinoderm, with a complete jointed stalk, is developed within the larva of Antedon. Is it not possible that the larva of Crossopodia may have developed a vermiform Echinoderm? With respect to the Phylogeny of the Arthropoda, I find myself disposed to take a somewhat different view from that of Professor Haeckel.
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.
The only modification which it occurs to me to suggest in this general view of the Phylogeny of the Vertebrata is, that the "Protamphirhine" was possibly more ganoid than shark-like.
We find an equally prolific branching of its two chief stems when we turn to the other division of the Indo-Germanic languages. The ancient Aryan gave rise to the numerous Iranian and Indian languages. This "comparative anatomy" and evolution of languages admirably illustrates the phylogeny of species.
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