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Updated: May 14, 2025
Seeing, for instance, that the oldest known mammals, reptiles, and fish strictly belong to their own proper classes, though some of these old forms are in a slight degree less distinct from each other than are the typical members of the same groups at the present day, it would be vain to look for animals having the common embryological character of the Vertebrata, until beds far beneath the lowest Silurian strata are discovered a discovery of which the chance is very small.
Furthermore, the earliest organisms had no hard skeletons, and it was not until living beings had developed far enough to have hard parts that it was possible for them to leave traces of themselves in the rocks. Hence, so far as concerns this earliest history, we can get no record of it in the rocks. ==Embryological.== But here comes in another source of evidence which helps to fill up the gap.
When we examine impartially the manuals of psychology that have been published by the most distinguished speculative philosophers and are still widely distributed, we are astonished at the naivete with which the authors raise their airy metaphysical speculations, regardless of the momentous embryological facts that completely refute them.
Each of these early embryological stages is represented by living animals, the undivided cell by the PROTOZOA, the blastosphere by some rare forms, and the gastrula in the essential structure of the COELENTERATES, the subkingdom to which the fresh-water hydra and the corals belong.
But embryological investigations have shown us that in the commencement none of these organs are formed, and yet that the principle of life is active, and that even after they exist, they cannot act, inclosed as they are.
After a careful description of the dorsal chord in its embryological development, he shows that a certain parallelism exists between the comparative degrees of development of the vertebral column in the different groups of fishes, and the phases of its embryonic development in the higher fishes.
It is interesting to note that the modern teleosts in their embryological growth pass through the stages which characterized the maturity of their Devonian ancestors; their skeleton is cartilaginous and their tail fin vertebrated. The Carboniferous system is so named from the large amount of coal which it contains.
Why should the embryo of any animal go through so many stages embryological allusions to forefathers of a widely different type? And why, again, should the germs of the same kind of creature always go through the same stages?
The generative idea resembles those concepts which, in the sciences, are of wide range because they condense a generalization rich in consequences. The subject is at first comprehended as a whole; development is organic, and we may compare it to the embryological process that causes a living being to arise from the fertilized ovum, analogous to an immanent logic.
Since we must choose from among these various anatomical views let us accept that of Flechsig, one of the most renowned and one having also the advantage of putting directly the problem of the organic conditions of the imagination. We know that Flechsig relies on the embryological method that is, on the development in the order of time, of nerves and centers.
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