308 THE ACTION OF NATURAL 
does any other individuals; yet children of the same 
parents are not all alike, and it often happens that 
they differ very considerably from their parents and 
from each other. This is equally true, of man, of all 
animals, and of all plants. Moreover, it is found that 
individuals do not differ from their parents in certain 
particulars only, while in all others they are exact 
duplicates of them. They differ from them and from 
each other, in every particular: in form, in size, in 
colour ; in the structure of internal as well as of external 
organs; in those subtle peculiaritics which produce 
differences of constitution, as well as in those still more 
subtle ones which lead to modifications of mind and 
character. In other words, in every possible way, in 
every organ and in every function, individuals of the 
same stock vary. 
Now, health, strength, and long life, are the results 
of a harmony between the individual and the universe 
that surrounds it. Let us suppose that at any given 
moment this harmony is perfect. A certain animal is 
exactly fitted to secure its prey, to escape from its 
enemies, to resist the inclemencies of the seasons, and 
to rear a numerous and healthy offspring. But a 
change now takes place. A series of cold winters, for 
instance, come on, making food scarce, and bringing 
an immigration of some other animals to compete with 
the former inhabitants of the district. The new immi- 
grant is swift of foot, and surpasses its rivals in the 
pursuit of game; the winter nights are colder, and 
require a thicker fur as a protection, and more 
