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At a lower psychic level, far back in our phylogenetic racial history, all of these movements, perhaps then in a rudimentary form, had a single, original meaning.
The ontogenetic facts that we gather from this sole survivor of the Acrania are the more valuable for phylogenetic purposes, as paleontology, unfortunately, throws no light whatever on the origin of the Vertebrates. Their invertebrate ancestors were soft organisms without skeleton, and thus incapable of fossilisation, as is still the case with the lowest vertebrates the Acrania and Cyclostoma.
In the Myxinoides the primitive kidneys retain this simple comb-shaped structure, and a part of it is preserved in the Selachii; but in all the other Craniotes it is only found for a short time in the embryo, as an ontogenetic reproduction of the earlier phylogenetic structure.
The phylogenetic significance of this fact, first emphasised by Huxley, is quite clear. The great question of the origin of the human race, or of "man's place in Nature," the "question of all questions," was then scientifically answered: "Man is descended from a series of ape-like Mammals."
The quickened pulse, the leaping heart, the accelerated respiration, the sighing, the glowing eye, the crimson cheek, and many other phenomena are merely phylogenetic recapitulations of ancestral acts. The thyroid gland is believed to participate in such physical activities.
By applying the principle of the discharge of nervous energy by phylogenetic association, and by making the additional hypothesis that in the discharge of nervous energy the thyroid gland is stimulated through the nervous system, we can explain many of the phenomena of Graves' disease and may possibly discover some of the factors which explain both its genesis and its cure.
By a number of important embryonic adaptations, the chief of which is the formation of an extensive food-yelk, the original course of the development of the vascular system has been so much falsified and curtailed in the higher Vertebrates that little or nothing now remains in their embryology of some of the principal phylogenetic features.
On this hypothesis we can understand the physical influence of one individual over the body and personality of another; and of the infinite factors in environment that, through phylogenetic association, play a role in the functions of many of our organs.
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.
These examples indicate how the history of the phylogenetic experiences of the human race may be learned by a study of the position and the action of the nociceptors, just as truly as the study of the arrangement and variations in the strata of the earth's crust discloses to us geologic history.
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