318 EMBRYOLOGY 



fitted for its place in nature, although it is clear that the fitness is 

 not in the organism, but in its interaction with its environment. 

 It is dependent and relative fitness, for an external change may 

 make unfit what before was fit. 



So far as the development of an embryo is a preparation for the 

 struggle of life, it is like the preparation of the kicked dog for further 

 violence. This struggle is in no way incompatible with the genera- 

 tion of sports and mutations and monsters and abortions and fail- 

 ures, but it is incompatible with the survival of those that fail in 

 the battle of life. 



What is true of development as a whole is also true of its suc- 

 cessive stages. So far as the interaction between each cell and 

 the internal conditions of its existence is a response to or a prepara- 

 tion for the next step in development, by changes of the same 

 general character as those that take place in the reproductive 

 organs of the bird and modify its plumage, and so far as the sum 

 of these changes fits the embryo for the state of life into which it is 

 to be born, it may survive and have descendants, but the embryo 

 that does not follow substantially the same course of development 

 as its allies is cut off from history. 



Since the germ-cells produced by an organism are, on the average, 

 more like those that produced it than like any other germ-cell, they 

 tend to follow a course of development which is practically but not 

 exactly the same. Since the struggle for existence is no philosophical 

 abstraction, no generalization, but a practical matter of personal 

 experience it does not depend, in any way, upon the causes or upon 

 the origin of the differences between the organism that survives 

 and the one that fails. It is therefore perfectly compatible with 

 the evidence that the germ-cells and the other cells of the body are 

 practically alike, and that the differences between them are not 

 inherent, but relative to and dependent upon the conditions of their 

 existence; nor is there any incompatibility between it and belief that 

 all cells and all organisms are practically so much alike that a new 

 animal kingdom might arise, in course of time, from a germ-cell of 

 any modern organism. 



So far as the germ-cell from which the new being arises, and the 

 cells that compose the tissues and organs of its parents, and the 

 germ-cells from which the parents arose, are practically alike, and 

 so far as the development of the new organism goes on under con- 

 ditions which are practically the same, on the average, as those 

 under which parents developed, the resemblance of child to parent, 

 which theories of heredity attempt to explain, is neither more nor 

 less than we might expect, and there is no problem, because all 

 experience teaches that similar bodies act and react in the same 

 way under similar conditions. 



