In a cow, subsisting on so innutritive a food as grass, we see that the immense quantity required necessitates an enormous digestive system; that the limbs, small in comparison with the body, are burdened by its weight; that in carrying about this heavy body and digesting this excessive quantity of food, much force is expended; and that, having but little remaining, the creature is sluggish.  Compare with the cow a horse—­an animal of nearly allied structure, but habituated to a more concentrated diet.  Here the body, and more especially its abdominal region, bears a smaller ratio to the limbs; the powers are not taxed by the support of such massive viscera, nor the digestion of so bulky a food; and, as a consequence, there is greater locomotive energy and considerable vivacity.  If, again, we contrast the stolid inactivity of the graminivorous sheep with the liveliness of the dog, subsisting on flesh or farinaceous matters, or a mixture of the two, we see a difference similar in kind, but still greater in degree.  And after walking through the Zoological Gardens, and noting the restlessness with which the carnivorous animals pace up and down their cages, it needs but to remember that none of the herbivorous animals habitually display this superfluous energy, to see how clear is the relation between concentration of food and degree of activity.

That these differences are not directly consequent on differences of constitution, as some may argue; but are directly consequent on differences in the food which the creatures are constituted to subsist on; is proved by the fact, that they are observable between different divisions of the same species.  The varieties of the horse furnish an illustration.  Compare the big-bellied, inactive, spiritless cart-horse with a racer or hunter, small in the flanks and full of energy; and then call to mind how much less nutritive is the diet of the one than that of the other.  Or take the case of mankind.  Australians, Bushmen, and others of the lowest savages who live on roots and berries, varied by larvae of insects and the like meagre fare, are comparatively puny in stature, have large abdomens, soft and undeveloped muscles, and are quite unable to cope with Europeans, either in a struggle or in prolonged exertion.  Count up the wild races who are well grown, strong and active, as the Kaffirs, North-American Indians, and Patagonians, and you find them large consumers of flesh.  The ill-fed Hindoo goes down before the Englishman fed on more nutritive food; to whom he is as inferior in mental as in physical energy.  And generally, we think, the history of the world shows that the well-fed races have been the energetic and dominant races.