i So Evolution and Adaptation 



those forms in which the females are fertilized during the 

 marriage flight. The explanation offered is that in these 

 forms the male carries the female, and this is assumed to 

 require greater size on his part. This loose way of guessing, 

 as to a possible explanation, is characteristic of the whole 

 hypothesis of sexual selection. First one, and then another, 

 guess is made as to the causes of the differences between the 

 sexes. It is not shown in a single one of the instances that 

 the postulated cause has really had anything to do with the 

 differences in question ; and the attempt to show that the 

 theory is probable, by pointing out the large number of cases 

 which it appears to account for, is weakened to a very great 

 degree by the number of exceptional cases, for which an 

 equally ready explanation of a different kind is forthcoming. 

 This way of giving loose rein to the imagination has been 

 the bane of the method that has followed hard on the track 

 of Darwin's hypothesis, and for which his example has been 

 in no small measure responsible. Thus, in the case just 

 quoted, there are no less than four distinct conjectures made 

 to account for the differences in size between the sexes, and 

 each guess involves an entirely different set of processes. 

 Considering the complicated relation of the life of organisms, 

 it may be doubted if any of the imagined processes could 

 bring about the result, and certainly not a single one has 

 been shown to be a real, or a sufficient, cause in the evolution- 

 ary process. Neither the actuality of the postulated causes, 

 nor their application to a particular case, has been shown 

 to exist. 



In the Diptera, or flies, Wallace records one interesting 

 case of sexual difference in the genus Elaphomyia of New 

 Guinea, in which the males are furnished with horns, which 

 the females lack. Darwin writes : — 



" The horns spring from beneath the eyes, and curiously 

 resemble those of a stag, being either branched or palmated. 

 In one of the species, they equal the whole body in length. 



