148 EVOLUTION OF LIFE. 
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, dark- 
haired, 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 varia- 
tions 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 Nutri- 
tion 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 
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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 
