DEGENERATION AND LAWS OF VARIATION. 203 



not be immediately detected. It is only after succes- 

 sive generations have been subjected to the change, 

 that its effect becomes apparent. In the second 

 place, the change is limited by the past history of 

 the species, by its constitution, the strength of its 

 hereditary impulse, and also by the amount and 

 rapidity of the changes which have been induced 

 during the time immediately preceding. Organisms 

 under changed conditions can only produce slight 

 variations of such structures as are already devel- 

 oped in the species. The changeability of any part 

 seems to be partly dependent upon the length of 

 time it has existed in the species. Darwin expressed 

 this fact and conclusion, as it appeared to him, by 

 saying that " characters become fixed by inheri- 

 tance," which means, that characters which have 

 appeared through many generations are less apt 

 to vary than perfectly new characters. When a 

 character appears and continues through many gener- 

 ations, its growth is due, not to a temporary, but to 

 a permanent, cause acting upon each individual. As 

 the growth of the peculiar character is repeated gen- 

 eration after generation, it becomes firmly associated 

 with the nervous co-ordinations controlling the gen- 

 eral growth of the organism, and forms part of the 

 inherited impulse. Naturally, therefore, characters 

 will vary less the longer they have been subjected 



