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From these pregnant facts of comparative anatomy and ontogeny it follows absolutely that the whole of the Mammals belong to a single natural stem, which branched off at an early date from the reptile-root. It follows further with the same absolute certainty that the human race is also a branch of this stem.
We have seen, in studying the evolution of the body as a whole, that phylogeny casts a light over the darker paths of ontogeny, and that we should be almost unable to find our way in it without the aid of the former. We shall have the same experience in the study of the organs in detail, and I shall be compelled to give simultaneously their ontogenetic and phylogenetic origin.
PHYLETICALLY. In accordance with the phylum or race; racially. PHYLETIC. Pertaining to a race or clan. PHYLOGENY. The history of the evolution of a species or group; tribal history; ancestral development as opposed to ontogeny or the development of the individual. PHYLUM. A term introduced by Haeckel to designate the great branches of the animal and vegetable kingdoms.
We should be quite unable to explain these if comparative anatomy and ontogeny did not come to our assistance. The vascular system in man and all the Craniotes is an elaborate apparatus of cavities filled with juices or cell-containing fluids. With the latter are connected the large cavities of the body, especially the body-cavity, or coeloma.
The shortest way to attain our purpose is that followed by Huxley in 1863 in his able work, which I have already often quoted, Man's Place in Nature the way of comparative anatomy and ontogeny.
Even if we entirely ignore it, all that we have learned from the zoological facts of comparative anatomy and ontogeny as to the placental character of man remains untouched. These prove beyond all doubt the common descent of man and all the rest of the mammals.
Comparative anatomy and ontogeny spoke too clearly for their testimony to be ignored any longer. But in order still to save man's unique position, and especially the dogma of personal immortality, a number of natural philosophers and theologians discovered an admirable way of escape in the "theory of degeneration."
The survival value of the slight gains in size and strength from millennium to millennium could have played no part. It was the indwelling necessity toward development that determined the issue. This assertion does not deliver us into the hands of teleology, but is based upon the idea that ontogeny and phylogeny are under the same law of growth.
In other words, the hypothesis explains heredity as part of the underlying problems of assimilation and of the causes which act directly during ontogeny.
But we have also many important grounds in comparative anatomy and ontogeny for assuming a common origin for all the Vertebrates. If the general theory of evolution is correct, all the Vertebrates, including man, come from a single common ancestor, a long-extinct "Primitive Vertebrate." Hence the genealogical tree of the Vertebrates is at the same time that of the human race.
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