OTHER FACTORS OF EVOLUTION 117 



them. Moreover not one of the many supporters of the theory 

 of germ-plasm has been able to make profitable use of it in the 

 twenty years since it was iirst published. On the other hand it 

 has had an evil influence in so far as it denied the inheriting of 

 acquired characters, which I hold, with Lamarck and Darwin, to 

 be one of the soundest and most indispensable supports of the 

 theory of descent."' 



There remain now for brief consideration three other factors 

 of Evolution, namely, (i) Sexual Selection, (2) the Effects of Use 

 and Disuse, and (3) the direct Influence of External Conditions ; 

 and in regard to each and all of them the degree of their influence 

 as factors hinges upon the all-important question which we shall 

 presently have to consider ; that is whether Acquired Characters 

 are or are not inherited.^ This latter question has long been 

 debated in relation to the last two of the factors just mentioned, 

 but it has only recently been very seriously considered in reference 

 to Sexual Selection. For this and other reasons it will be con- 

 venient briefly to consider and make some statements in reference 

 to this factor before dealing with the other two. 



In Darwin's view the process of Sexual Selection was essenti- 

 ally similar to that of Natural Selection, in fact, a corollary 

 therefrom, and consequently explicable, with the necessary varia- 

 tions, in much the same kind of way. This view, however, has 

 not been generally accepted. It has been rejected by Wallace, 

 for instance, who instead of dwelling upon contests between males 

 and the effects of natural selection, combined with heredity, in 

 perpetuating the characters of the victors — whether in the 

 direction of defence or of allurement — considers that the principal 

 cause of the greater brilliancy of male animals in general, and 

 of male birds in particular, is, that they do not stand so much 

 in need of protection by concealment as the females do. For the 

 rest he relies in the main simply upon Natural Selection. 



Several other theories in connection with this subject are 

 discussed by J. T. Cunningham in his interesting work "Sexual 



' " The Wonders of Life," Transln., 1904, p. 104, see also pp. 385 and 391. 



' In many collateral respects also this question is one of enormous 

 importance, as may be gathered when Herbert Spencer says : " More than 

 once I have pointed out that, as influencing men's views about Education, 

 Ethics, Sociology, and Politics, the question whether acquired characters are 

 inherited is the most important question before the scientific world " (" Prin- 

 ciples of Biology," I, p. 672). 



