I 



Heredity and the Origin of Variations, 151 



There is no increase in the set of organs a, which are 

 strongly developed in both parents ; and no decrease in 

 the set of organs e, which are weakly developed in both 

 parents. By sexual admixture alone there can be no 

 increase or decrease beyond the mean of the two parental 

 forms. If, then, the union of sperm and ovum be the 

 source of new or more favourable variations other than or 

 stronger than those of either parent, this must be due to 

 the fact that the hereditary tendencies not merely com- 

 mingle, but under favourable conditions combine, in some 

 way different indeed from, but perhaps analogous to, that 

 exemplified in chemical combination. 



Such organic combination, as opposed to mere com- 

 mixture, is altogether hypothetical, but it may be worth 

 while to glance at some of its implications. If it be 

 analogous to chemical combination, the products would 

 be of a definite nature; in other words, the variations 

 would be in definite directions. Selection and elimination 

 would not have to deal with variations in any and all 

 directions, but would have presented to them variations 

 specially directed along certain lines determined by the 

 laws of organic combination. As Professor Huxley has 

 said, "It is quite conceivable that every species tends to 

 produce varieties of a limited number and kind, and that 

 the effect of natural selection is to favour the development 

 of some of these, while it opposes the development of others 

 along their predetermined line of modification." Mr. 

 Gulick * and others have been led to believe in a tendency 

 to divergent evolution residing in organic life -forms. Such 

 a tendency might be due to special modes of organic com- 

 bination giving rise to particular lines of divergence. 

 Again, we have seen that some naturalists believe that 

 specific characters are not always of utilitarian significance. 

 But, as was before pointed out, on the hypothesis of all- 

 is that sexual admixture tends towards the mean of the race (ancestors 

 included), and cannot be credited with new and unusually favourable variations. 

 The prepotency of one parent is also here neglected. 

 * See his valuable paper on " Divergent Evolution," Lin. Soc. Zool., No. exx. 



