180 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 bea 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. 
