Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/198

148 The Edentata are so called from the peculiar character of their teeth; but the skin-covering of many of them, like the Armadillo and Pangolin, is equally remarkable. Short and compressed heads accompany short limbs, as seen in Pigs and Cattle. The horned animals are without incisor and canine teeth, as seen in the Ruminants, etc.; while those that have these teeth (like Pigs and the Musk-deer) never exhibit horns. With the long legs of Wading-birds (Heron and Stork) are associated long beaks. Dark-skinned, darkhaired, and brown-eyed Europeans are less susceptible to tropical diseases, and therefore more easily acclimatized, than those with light skin, blond hair, and blue eyes. These examples illustrate the important principle of one variation entailing another through the correlation of organs. The different modifications that we have mentioned are variations appearing in the parent, and often transmitted to the offspring. But there are also variations which first appear in the posterity, such as monstrosities, the difference of the sexes, etc. Of these variations it is often difficult to say whether they are produced by causes acting directly on the parents, or directly on posterity, or indirectly through the parents on posterity. While the causes of many variations as well as their transmission to posterity are obscure or unknown, it seems very probable that changes in Nutrition are the causes of all variations, the term Nutrition including the effects of Food, Climate, Social Relations, Use and Disuse, Correlation of Organs, etc.; while the facts of Inheritance are to be explained by the laws of Generation, of which as yet few are known. No doubt at some future day Nutrition and Generation will be shown to be simply physical and chemical phenomena. However that may be, the important fact is that "all organic individuals become in the course of their life, through adaptation to different conditions of existence, unlike one another, although the