VIIL THE DEVELOPMENT OF HUMAN RACES 175 
or totally fails, it can only exist by becoming adapted toa new 
kind of food, a food perhaps less nourishing and less digestible. 
Natural selection will now act upon the stomach and intes- 
tines, and all their individual variations will be taken advan- 
tage of, to modify the race into harmony with its new food. 
In many cases, however, it is probable that this cannot be 
done. The internal organs may not vary quick enough, and 
then the animal will decrease in numbers and finally become 
extinct. But man guards himself from such accidents by 
superintending and guiding the operations of nature. He 
plants the seed of his most agreeable food, and thus procures 
a supply, independent of the accidents of varying seasons or 
natural extinction. He domesticates animals, which serve him 
either to capture food or for food itself, and thus changes of 
any great extent in his teeth or digestive organs are rendered 
unnecessary. Man, too, has everywhere the use of fire, and 
by its means can render palatable a variety of animal and 
vegetable substances, which he could hardly otherwise make 
use of, and thus obtains for himself a supply of food far 
more varied and abundant than that which any animal can 
command. 
Thus man, by the mere capacity of clothing himself, and 
making weapons and tools, has taken away from nature that 
power of slowly but permanently changing the external form 
and structure in accordance with changes in the external 
world, which she exercises over all other animals. As the 
competing races by which they are surrounded—the climate, 
the vegetation, or the animals which serve them for food—are 
slowly changing, they must undergo a corresponding change 
in their structure, habits, and constitution to keep them in 
harmony with the new conditions—to enable them to live 
and maintain their numbers. But man does this by means 
of his intellect alone, the variations of which enable him, with 
an unchanged body, still to keep in harmony with the changing 
universe. 
There is one point, however, in which nature will still act 
upon him as it does on animals, and, to some extent, modify 
his external characters. Mr. Darwin has shown that the colour 
of the skin is correlated with constitutional peculiarities both 
in vegetables and animals, so that liability to certain diseases 
