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 ether. 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 peculiarities 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 



