THE THEORY 3 
but sex and age are by far the most im- 
portant. 
The next fact that attention is drawn to, 
is that within a species individuals often differ 
widely in structure and characters. Males 
differ from females, young from old, in form, 
colour, structure, habits, instincts, and in many 
other characters apart from their sexual organs. 
These differences within a species are ap- 
parently often greater than the differences 
which distinguish one species from another 
(compare male and female pheasant with 
House and Tree sparrows), and are infinitely 
greater than the variations always to be found 
in every species, and by means of which, ac- 
cording to Darwin’s theory of the Origin of 
Species, evolution has taken place. 
Attention to one other fact must be called, 
—namely, that individuals of a species often 
form themselves into societies: males and 
females become mated and form pairs, which 
keep together throughout the breeding season, 
or it may be for life. Parents and young also 
often keep together, forming families; or 
families may meet to form herds; societies 
which may be temporary or permanent. More 
complicated societies are not unknown; for 
instance, the hive bees. 
