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The peculiar embryonic development of the vertebrate eye does not enable us to draw any definite conclusions as to its obscure phylogeny; it is clearly cenogenetic to a great extent, or obscured by the reduction and curtailment of its original features. It is probable that many of the earlier stages of its phylogeny have disappeared without leaving a trace.
It follows at once, as regards the phylogeny of the apes, that two divergent lines proceeded from the common stem-form of the ape-order in the early Tertiary period, one of which spread over the Old, the other over the New, World.
Even Professor Haeckel, of the famous University of Jena, would not deny this, with all that his new terms "ontogeny" and "phylogeny" may imply. When potential life passed over into actual life in the individual Plato, it was not the pabulum that assimilated the man, but the man the pabulum.
Gentlemen, just bridge one gulf for us; the gulf lying between any two species will do. We get impatient, standing and gazing. Look! Can you see across? Mr. Darwin says, "Professor Haeckel, in his general Morphology and other works, has brought his great knowledge and abilities to bear on what he calls phylogeny or the lines of descent of all organic beings." Origin of Species, p. 381.
If we examine this undoubted fact from the point of view of phylogeny, in the light of the theory of descent, it follows at once that man is of a common stem with all the other Mammals, and comes from the same root as they. But the various features in which the Mammals agree and by which they are distinguished are of such a character as to make a polyphyletic hypothesis quite inadmissible.
The origin and development of the blastoderm or germinal disc has nothing to do with recapitulation of the phylogeny. It is not the case that the embryos of higher vertebrates are indistinguishable from one another. Even the egg-cell has a specific character, and is totally different from any unicellular organism at the Protistan level.
When I made the first attempt in my General Morphology to work out the theory and apply it to classification, I found no problem of phylogeny that gave me so much trouble as the linking of the vertebrates with the invertebrates. But just at this time the true link was discovered, and at a point where it was least expected.
The contrast between a man whose associational centers are keen and a man in whom these centers are dulled or lost is the contrast between life and death. In accordance with the law of adaptation through natural selection, phylogeny, and association, one would expect no pain in abscess of the brain, in abscess of the liver, in pylephlebitis, in infection of the hepatic vessels, in endocarditis.
In every case the duration of ontogeny shrinks into insignificance when we compare it with the enormous period that has been necessary for phylogeny, or the gradual development of the ancestral series. This period is not measured by years or centuries, but by thousands and millions of years.
According to this view one cell accidentally developed the attributes of vegetable life; a further accident leads another cell to initiate the line of invertebrates; another that of fishes, let us say; another of mammals: the number varying according to the views of the theorist on phylogeny.
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