THEORIES OF HEREDITY 115 



is different from what it would have been before the 

 modification ever occurred. 



"On the one hand," says Spencer, "physiological 

 units will, because of their special polarities, build 

 themselves into an organism of a special structure ; so, 

 on the other hand, if the structure of this organism is 

 modified by modified function, it will impress some 

 corresponding modification on the structures and po- 

 larities of its units. . . . If the aggregate is made 

 by incident actions to take a new form, its forces must 

 tend to re-mould the units into harmony with this new 

 form. And then, these units, when separated in the 

 shape of reproductive centres, will tend to build them- 

 selves up into an aggregate modified in the same di- 

 rection." 1 



This explanation is purely theoretical. What 

 Spencer tries to do is not so much to determine 

 through what physiological process such a repercus- 

 sion becomes possible, as to make this repercussion 

 tally with certain fundamental principles like the 

 principle of the persistence of force. He explains 

 that in the course of one individual's life "any 

 functional and structural divergence produced by 

 a new incident action, would increase until the 

 new incident action was counterpoised ; but nothing 

 changes the fact that the replacing of a continu- 

 ously existing individual by a succession of individ- 



i Principles of Biology, Vol. I, p. 319. 



