Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/196

146 expression, however, is not absolutely correct, since the members of every family differ more or less in the color of their eyes and hair, in their complexion, dispositions, etc. The same species of trees differ as regards the size of their stem, number of their branches, leaves, flavor of their fruit, etc. Let us examine now, a little in detail, some of these variations, and attempt to indicate their probable causes.

The quantity and quality of the food are known to modify animals and plants. Thus, what a marked difference is produced in the habit of our domestic animals, who are fed daily, and of wild ones, whose means of subsistence are so precarious! The quality of the food modifies, as well as the quantity, the tissues: thus, richly azotized food develops little fat, poorly azotized food, on the contrary, a great deal. The farmer requiring fine wool supplies his sheep with different food from that which he gives wishing to obtain good meat. Notice the effects of a rice diet as seen on the Chinese, and of a beef one on the English. Climate is an important element in the production of variations: thus, plants growing in dry, warm, and sunny places offer a very different aspect from those of moist, cool, and shady spots. Plants that at the sea-side exhibit thick, fleshy leaves, in hot, dry places develop haired ones. The crowding of trees has the effect of making the stem tall, while it diminishes the foliage; whereas the foliage of the isolated tree expands, the stem being comparatively short. We see, therefore, that the social state, so to speak, is of importance in modifying forms. The use and disuse of organs produce most marked effects: thus, the wings of the domestic duck are lighter than those of the wild one, whereas the legs of the domestic duck are heavier; the difference being caused undoubtedly by the different habits