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This process is continuously repeated is simultaneously going on in all parts of the growing embryo; and by endless such differentiations there is finally produced that complex combination of tissues and organs constituting the adult animal or plant. This is the history of all organisms whatever.
As a consequence of these ceaseless differentiations, we now find that no considerable portion of the Earth's exposed surface is like any other portion, either in contour, in geologic structure, or in chemical composition; and that in most parts it changes from mile to mile in all these characters. Moreover, there has been simultaneously going on a differentiation of climates.
Darwin, differentiations of structure and instinct have mainly come about through the accumulation of small, fortuitous variations without intelligence or desire upon the part of the creature varying; modification, however, through desire and sense of need, is not denied entirely, inasmuch as considerable effect is ascribed by Mr.
Obviously, too, pursuance of inquiries of the kind indicated, raises questions which science is not yet prepared to answer; as, for instance Whether all nervous functions, in common with all other functions, arise by gradual differentiations, as their organs do? Whether the emotions are, therefore, to be regarded as divergent modes of action that have become unlike by successive modifications?
Let the first periodically recurring substance we will say A be able to recur or reproduce itself, not once only, but many times over, as A1, A2, &c.; let A also have consciousness and a sense of self-interest, which qualities must, ex hypothesi, be reproduced in each one of its offspring; let these get placed in circumstances which differ sufficiently to destroy the cycle in theory without doing so practically that is to say, to reduce the rotation to a spiral, but to a spiral with so little deviation from perfect cycularity as for each revolution to appear practically a cycle, though after many revolutions the deviation becomes perceptible; then some such differentiations of animal and vegetable life as we actually see follow as matters of course.
And the Earth was a monstrous, incredible, abysmal blackness which at this first experience of its appearance produced an almost superstitious terror. Formerly it had seemed a distant but sunlit world, flecked with white clouds and with sprawling differentiations of color beneath them. Now it did not look like a solid thing at all. It looked like a hole in creation.
The Darwinian theory, the interpretation of the teleological in the animate world by means of the theory of descent based upon natural selection, entered like a ferment into the scientific thought-movement, and in a space of forty years it has itself passed through a series of stages, differentiations, and transformations which have in part resulted in the present state of the theory, and have in part anticipated it.
But does it follow that every degree, every shade, every detail of sensation, even the most insignificant, has any importance for the action? The variations of sensibility are much more numerous than those of movements and of adaptation; very probably, as is seen in an attentive study of infancy, sensibility precedes the power of motion in its differentiations.
In the Hydra, the respective parts of the creature's substance have many functions in common. They are all contractile; omitting the tentacles, the whole of the external surface can give origin to young hydræ; and, when turned inside out, stomach performs the duties of skin and skin the duties of stomach. In aboriginal societies such differentiations as exist are similarly imperfect.
Southard obtained his differentiations purely from pathological cases or whether, accepting Shand or Pound or both, using their distinctions as apperceptive organs, he unconsciously reads their distinctions into his cases. His paper, at any rate, is a genuine contribution as well as an encouragement to those who seek to correlate the normal with the abnormal.
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