6 CONTROLLED NATURAL SELECTION 
although apparently similar, they are in fact 
not so; then the chance of selecting one out 
of male and female birds must also not be 
half; and in this case, because the individuals 
are markedly different in structure, it is likely 
to be further removed from half than in the 
case of the apparently similar hens. 
Thus these great differences in structure 
between male and female, young and old, 
must cause the male and the old to be more 
or less liable to be selected than the female 
and the young. One cannot conceive these 
individuals having an equal chance of survival 
in the struggle for existence. 
Continuing this conception further, a ques- 
tion at once arises: May not these differ- 
ences in structure between male and female, 
young and old, be such, that the more valuable 
females and young may have a better chance 
of survival than the less valuable males and 
old? may not this be their purpose? This 
question, although asked now, is answered 
under heading 4, and is best answered after 
headings 2 and 3 have been discussed. 
2. Natural Selection must treat associations 
of individuals as units, just as it does indi- 
viduals. If, within a species, owing to the 
struggle for existence, the fittest individuals 
