88 SCIENCE PROGRESS 



variation being then only the expression in plants and animals 

 of an infinite differentiation common to organic and inorganic 

 nature, it is idle to search for its ultimate causes in phenomena 

 which are subsequent to life. Sex has been suggested as a 

 cause, but is itself a variation of life, as life itself is but a 

 variation of inorganic nature, and therefore it cannot be the 

 cause. The effects of the environment upon a parent, supposed 

 on very inadequate evidence to be transmitted to their off- 

 spring, have been pressed into service as causing or contributing 

 to cause variation. But there is no need to spread the net so 

 wide, and it is unreasonable in accounting for a phenomenon 

 everywhere manifested throughout the universe to bring in 

 causes peculiar to life. Variation, differentiation, the produc- 

 tion of the unlike out of the like, is a part of the universal 

 scheme of things. The power to vary is a gift passed on to 

 the organic world by the inorganic from which it sprang. The 

 word differentiation was at first part of the formula in which 

 Spencer summed up the course of evolution, and though it 

 was afterwards discarded, its essence remains, and it seems 

 questionable whether the formula gained by the change. If one 

 word could sum up the process of the suns from a homogeneous 

 nebula to the complexity of mundane affairs, that word would 

 be differentiation. 



The ultimate cause therefore of organic variation is the 

 same as that of differentiation in general. Variation is a phase 

 of the phenomenon called by Spencer the instability of the 

 homogeneous — the tendency of like things to become unlike, 

 and of unlike things to become more unlike. It may accord- 

 ingly well be questioned whether the object of our search ought 

 not, instead of the cause of variation, to be the cause of 

 similarity. How is that wonderful constancy to type in all 

 that is essential to life preserved in the immensely complicated 

 organisms of the higher animals ? It is a question that is to 

 some extent being answered by the researches of cytologists 

 on the growth and division of cells. 



All this however is naturally unsatisfying to the biologist, 

 who wishes to trace not merely the ultimate, but also the 

 proximate cause of variation in the organic world. How is this 

 universal tendency to change and divergence manifested in the 

 world of life, and especially in the higher animals ? I believe 

 it is possible by consideration of the mechanics of reproduction 



