THE METABOLISM OF THE BODY. 469 
natural, self-chosen diet may be varied when confined, will 
be astonished at the plasticity of their instincts, usually con- 
sidered as so rigid in regard to feeding. These facts help 
one to understand how by the law of habit and heredity 
each group of animals has come to prefer and flourish best 
upon a certain diet. But habit itself implies an original 
deviation some time, in which is involved, again, plasticity 
of nature and power to adapt as well as to organize. With- 
out this, evolution of function is incomprehensible; but with 
this principle, and the tendency for what has once been done 
to be easier of repetition; and, finally, to become organized, 
a flood of light is thrown upon the subject of diet, diges- 
tion, and metabolism generally. On these principles it is 
possible to understand those race differences, even individ- 
ual differences, which as facts must be patent to all observ- 
ers. Every individual’s own history will teach him that he 
can learn to digest and assimilate what was once all but a 
poison to his organism; so that it becomes comprehensible 
how a Chinaman, for example, can, not only remain in health, 
but do a large amount of work daily on a diet on which the 
ordinary Englishman might ae starve before he could 
adapt himself to it. 
It is also a well-established fact hat whole families crave 
and seem to require certain articles of diet in excess, as com- 
pared with the majority—e. g., a meat diet. In some in- 
stances, at all events, this can be traced to pathological excess 
in the ancestors. It is important to recognize, however, that 
while such a diet upon the whole may be the best that can be 
appropriated at the time, itis associated with certain aberrations 
of function which it is desirable to correct; hence the wisdom 
of withholding from such people, even children, to a certain 
extent, the meat which they so much crave. The habit of the 
metabolism may be modified. The rapid rate of speed of the 
metabolic processes, which an excess of such a diet is apt to 
beget, leads to various bad results, such as great irritability of 
the nervous system, and a general lack of stability and equi- 
poise in the vital machine. 
The principle of natural selection has clearly played a great 
part in determining the diet of a species; the surviving emi- 
grants to a new district must be those that can adapt to the local 
environment best, including the food which the region supplies. 
The greater capability of resisting hunger and thirst in some 
individuals of a species implies great differences in the meta- 
