The Facts of Sex in Relation to Metabolism. 281 
great amount is used for the formation of ova, or for the 
nutrition of the embryo. The female organism is one in 
ut ak. cilasil. 
which the general ratio K (the mean of many ratios) is 
greater than the corresponding general ratio in the male. 
(c) As to the origin of sex, which we take to be the origin 
of two contrasted physiological habits, we adhere to what is 
suggested in “The Evolution of Sex,” and virtually accepted 
by Hartog (see p. 269). We believe that the origin of sex is 
clearly illustrated by the history of such forms as Pandorina 
and Volvoz. It is of course possible, and indeed necessary, 
to push the inquiry a step further back, seeking some 
explanation why a Pandorina should produce mega-, meso-, 
and micro-gametes, or why a Volvoxz should be partheno- 
genetic, hermaphrodite, female, or male, as the case may be. 
To this question it does not seem to us possible to give any 
answer other than that which refers the diverse physiological 
states to diverse environmental conditions. The question is 
in fact as ultimate as that which asks how animals and 
plants arose as divergent branches of the tree of life. 
As to the reasons why the sexual dimorphism should be 
fostered in the struggle for existence by the usual process of 
natural elimination, there is, as the preceding reviews will 
show, considerable difference of opinion. According to 
Weismann, all the facts of sex and sexual reproductions. 
are adaptations to secure amphimixis, or the mingling of 
individual characteristics, whence arises the crop of variations 
which natural selection thins. According to others, who 
believe in rejuvenescence theories of fertilisation, the 
dimorphism secures the continual formation of new physio- 
logical associations or of new chemical combinations. 
(d) While it is possible for any one to suppose that all the 
characters of the sexes are purely adaptive, arising from 
indefinite variations reduced to advantageous lines by 
natural elimination (or selection), and while we ourselves 
are compelled to adopt a similar explanation in regard to 
certain exceptional facts of sex, such as the fact that many 
male mammals are larger than their mates, the following 
considerations may be noticed :— 
