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 



