4 CONTROLLED NATURAL SELECTION 
On these three facts the theory may be said 
to rest : 
1. Within a species, individuals differ in 
value. 
2. Within a species, individuals differ in 
structure. 
3. Within a species, individuals form socie- 
ties. 
Further, there is a correlation between these 
facts, in that the differences in value are 
associated with the differences in structure. 
Females are more valuable than males ; females 
differ in structure from males other than in 
their sexual organs. Young are more valuable 
than old, and they too are different in structure. 
The third fact is also correlated, in that differ- 
ence in structure between male and female 
and between young and old are especially to 
be found in those animals which are closely 
united socially, either sexually or into families. 
The theory can now be stated under four 
headings : 
1. Natural Selection appreciates the differ- 
ences in character which distinguish male from 
female, young from old. 
2. Natural Selection must treat associations 
of individuals as units, just as it does single 
individuals. 
