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. 



