146 EVOLUTION OF IER 
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. 
VARIATIONS. 
The quantity and quality of the food are known to mod- 
ify 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 crowd- 
ing 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 im- 
portance 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 
