THE THEORY 11 
3. Individuals differ widely in structure : 
male from female, young from old. 
4. Correlation of these three facts: unequal 
value with difference of structure, and both 
with the formation of societies. 
Sequence of argument : 
1. Natural Selection must appreciate differ- 
ences in structure ; 
2. Natural Selection must treat associations 
as units ; 
3. And bring about diversity of structure 
in them ; 
4, Which structures control Natural Selec- 
tion, so that less valuable individuals are more 
liable to be destroyed than more valuable. 
Thus when male is found to be different 
from female it is necessary to inquire whether 
the difference be due to (1) a character of 
the individual which fits a monosexual en- 
vironment, or (2) a character of the pair, also 
of cause an environmental character, that 
controls Natural Selection. And so with all 
structures occurring in sociable animals and 
confined to particular individuals. 
Thus it is as a working hypothesis that this 
new theory is brought forward, in the hope 
that it may possibly explain the origin and 
use of some polymorphisms. 
