8 CONTROLLED NATURAL SELECTION 
of himself, as any of his personal characters, 
re evolution. 
Those structures or characters of the indi- 
viduals forming the society, which are alike, 
being common to all, can of course be con- 
sidered as units if desirable; but those struc- 
tures or characters peculiar to, say, the male 
of a pair or of a family, cannot and must not 
be considered apart from the pair or the 
family. 
This conception has been carried thus far, 
in order that the explanation of heading 3 
may be more easily understood. 
3. Natural Selection, &c.—If the fixation of 
a useful variation in an individual is brought 
about by heredity and Natural Selection, then 
when a useful variation occurs in a society, 
although it may be confined to only one class 
of individuals and not be found in all the 
classes of individuals forming the society, it 
none the less will become fixed in a like 
manner. 
For instance, certain males of a species may 
develop, as a variation, a certain character 
which, although not beneficial to them, or it 
may even be harmful to them, is, however, 
useful to the pair, male and female; enabling 
the pair to survive in the struggle for exist- 
