316 EMBRYOLOGY 



they recede by variation, nor are they standards fixed in living 

 beings by inheritance. 



No two objects, alive or dead, ever are exactly alike, and as we 

 all know that life is a struggle, and admit the diversity of nature, 

 neither the stability of species nor the mutability of species is any- 

 thing more than we might expect. Inheritance and variation are 

 not two things, but two imperfect views of a single process, for 

 the difference between them is neither in living beings nor in any 

 external standard of extermination, because it is in the inter- 

 action between each living being and its competitors and enemies 

 and sources of food and other necessaries of life. It would be idle 

 to seek, within the germ, or in the conditions of its existence, for a 

 principle of inheritance, or for the cause of variation, because species 

 is in the interaction between the organism and its environment. 



The specific stability of a growing embryo is no more separable 

 from its individuality than the height of a man is separable from 

 the man. We can think of the height of men without thinking of 

 the men, and we can tabulate statures and treat them by statistical 

 methods, but, while we may, for some purpose that we have in 

 view, withdraw our attention from the men, each height still remains 

 the height of an individual and particular man. So, too, we may 

 tabulate the differences between animals without thinking of their 

 kinship, or we may tabulate their resemblances without thinking 

 of their individuality, but this is no more reason for thinking 

 inheritance and variation are two things than for thinking a man 

 and his stature are two things, or for thinking the head of a man is 

 anything else than a man's head. 



Individual dogs are more like other dogs than they are like any- 

 thing else in nature, and yet one dog is different from another. Is 

 there any reason for thinking the case of cells is any different? 

 Daughter cells may be different, even when they are more like each 

 other than they are like any other cell. The difference between 

 homologous twins and ordinary twins, or puppies of the same litter, 

 shows that germ-cells are not identical, and this is shown, still more 

 clearly, by experiments like those of Mendel. The fact that cells 

 are different is no more proof that they are specifically differen- 

 tiated than the difference between dogs is proof that the races of 

 dogs were foreordained. 



Cell-division may be neither integral nor differential, for the 

 sameness, or kinship, of cells is not philosophical or abstract identity, 

 but practical equivalence in the economy of the organism. When 

 we say two cells are the same in substance, I cannot discover 

 that we mean anything more than our meaning when we say- the 

 report of a conversation is the same in substance as the original 

 conversation. 



