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During the years in which he had been growing into manhood he had come instinctively into sound notions of what was artistic and refined. He had seen so many homes that were more distinguished and harmonious than his own. One could not walk or drive about Philadelphia without seeing and being impressed with the general tendency toward a more cultivated and selective social life.
But the agent of this selective process is the struggle for existence, which is a no less important factor. Thus Evolution depends on these three laws: Tendency to vary, or variation, natural selection, and struggle for existence. Science tries to explain through these three laws the physical, mental, intellectual, moral and spiritual evolution of mankind.
We have said that the following out of the properties exhibited by physiological species would lead us into difficulties, and at this point they begin to be obvious; for, if, as a result of spontaneous variation and of selective breeding, the progeny of a common stock may become separated into groups distinguished from one another by constant, not sexual, morphological characters, it is clear that the physiological definition of species is likely to clash with the morphological definition.
For the picture presented by an infection, temperature, rash, prostration, are the details of the general reaction of the organism in the face of a new situation, the presence of a powerful, destructive invader. Information has accumulated that the invader is powerful and destructive, as well as selective, because of endocrine deficiency of one sort or another in the body it has attacked.
Darwin had become, as he himself says, a veritable Croesus, "overwhelmed with his riches in facts" facts of zoology, of selective artificial breeding, of geographical distribution of animals, of embryology, of paleontology. He had massed his facts about his theory, condensed them and recondensed, until his volume of five hundred pages was an encyclopaedia in scope.
But as contemplation is actually a luxury in our lives, and things interest us chiefly on passionate and practical grounds, the accumulation of values too exclusively aesthetic produces in our minds an effect of closeness and artificiality. So selective a diet cloys, and our palate, accustomed to much daily vinegar and salt, is surfeited by such unmixed sweet.
There is, in fact, one set of these peculiarities which the theory of selective modification, as it stands at present, is not wholly competent to explain, and that is the group of phenomena which I mentioned to you under the name of Hybridism, and which I explained to consist in the sterility of the offspring of certain species when crossed one with another.
So far as regards the selective conservation of capacities or aptitudes in individuals, these two lines may be called the pecuniary and the industrial. As regards the conservation of propensities, spiritual attitude, or animus, the two may be called the invidious or self-regarding and the non-invidious or economical.
Darwin has satisfactorily proved that what he terms selection, or selective modification, must occur, and does occur, in nature; and he has also proved to superfluity that such selection is competent to produce forms as distinct, structurally, as some genera even are. If the animated world presented us with none but structural differences, I should have no hesitation in saying that Mr.
"Our acceptance of the Darwinian hypothesis must be provisional so long as one link in the chain of evidence is wanting; and so long as all the animals and plants certainly produced by selective breeding from a common stock are fertile with one another, that link will be wanting." In a recent book, "Creation or Evolution?
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