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Man shares all the characteristics I have described with all the Mammals, and differs in them from all other animals. Finally, from these facts we deduce with the same confidence those advances in the vertebrate organisation by which one branch of the Sauromammals was converted into the stem-form of the Mammals.
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.
In the human female there is usually only one pair of glands, at the breast; and it is the same with the apes, bats, elephants, and several other mammals. This polymastism points back to an older stem-form. Sometimes, moreover, the normal mammary glands are fully developed and can suckle in the male; but as a rule they are merely rudimentary organs without functions in the male.
While the cutaneous glands are inner growths of the epidermis, the appendages which we call hairs and nails are external local growths in it. The lower mammals usually have claws instead of them; the ungulates, hoofs. The stem-form of the mammals certainly had claws; we find them in a rudimentary form even in the salamander.
Undoubtedly both the Tunicates and Acrania have inherited the chorda from a common unsegmented stem-form; and these ancient, long-extinct ancestors of all the chordonia are our hypothetical Prochordonia.
Just as the countless species of the Metazoa do actually develop in the individual from the simple embryonic form of the gastrula, so they have all descended in past time from the common stem-form of the Gastraea.
Some forms have diverged more, and some less, from the original stem-form. This easily demonstrable fact illustrates very well the analogous case of the origin of the vertebrate species. Phylogenetic comparative philology here yields a strong support to phylogenetic comparative zoology.
The previous chapters have taught us how the human body as a whole develops from the first simple rudiment, a single layer of cells. The whole human race owes its origin, like the individual man, to a simple cell. The unicellular stem-form of the race is reproduced daily in the unicellular embryonic stage of the individual.
There is absolutely no reason why there should be five toes in the fore and hind feet in the lowest Amphibia, the reptiles, and the higher Vertebrates, unless we ascribe it to inheritance from a common stem-form. Heredity alone can explain it. It is true that we find less than five toes in many of the Amphibia and of the higher Vertebrates.
All these living forms have diverged more or less from the ancestral form; none of them could engender the same posterity that the stem-form really produced thousands of years ago. It is certain that man has descended from some extinct mammal; and we should just as certainly class this in the order of apes if we had it before us.
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