282 Proceedings of the Royal Physical Society. 
(1) Even in regard to the secondary sexual characters, 
which have been and are still regarded by many as good 
illustrations of the results which may be achieved by the 
operation of natural elimination on an abundant crop of 
indefinite variations, Wallace’s criticism of Darwin’s theory 
of sexual selection has reopened the question, while his own 
physiological explanation is parallel with that suggested in 
“The Evolution of Sex” for the general facts of sexual 
dimorphism. 
(2) Sexual dimorphism appears in many forms among 
organisms, and those who would explain it as purely 
adaptive, are driven to give as many special explanations in 
terms of adaptation as there are varieties of reproductive 
habit, while the single “ constitutional theory” covers nearly 
all known cases. 
(3) Further, it is to us impossible to conceive of the way 
in which a favourable feminine variation, such as size suit- 
able for nurturing an embryo, can always have been entailed 
upon the female sex. It is true that it is easy to say that 
those species and individuals in which this mode of entail or 
inheritance was not established would be eliminated, still 
the difficulty remains, how in any case is the entail kept up 
if it be not the expression of a constitutional characteristic. 
(e) Finally, it may be noted that the suggestions discussed 
in the present paper are in harmony with many other 
attempts which are now being made from different sides to 
prosecute an analysis beneath the possibilities of elimination 
or selection to the origin of the variations themselves. And 
if in regard to this fundamental problem we have been able 
to say but little, we may perhaps have succeeded in emphasis- 
ing the fact that the problem has to be faced. It is indeed 
to some lack of perception of the fact that primary factors, 
originating variation, must lie behind the secondary factors, 
directing variation, that we must refer the misunderstandings 
which continue to prevail in the camps of Neo-Lamarckians 
and Darwinians. 
