140 ESSAYS OF A BIOLOGIST 



systems, in the form of still mechanical instincts, or 

 they may be concerned in some way or other with the 

 emotional side of the animals, and consist in characters 

 and actions which stimulate the emotions of the other 

 sex, characters which we have termed epigamic. 



It is only in higher groups that these emotion- 

 stimulating sexual characters arise, for only in them 

 has mind reached a sufficient degree of perfection. 

 But even though detailed study reveals in a bird or 

 a mammal a mental life of a complexity far more 

 considerable than the average man would imagine, 

 yet on the whole it is straightforward and its currents 

 run fairly direct from stimulus to fulfilment. 



When we reach man, however, the whole aspect 

 of the matter changes. The change is most marked, 

 naturally, in his mental organization. Through his 

 powers of rapid and unlimited association, any one 

 part of his experience can be combined with any 

 other ; through his powers of generalizing and of 

 giving names to things, his experience is far more 

 highly organized than that of any animal ; through 

 speech and writing he is inheritor of a continuous 

 tradition which enormously enlarges his range of 

 experience. Again, he can frame a purpose and 

 thus put the objective of his actions far further into 

 the future than can lower organisms. 



There are, however, also changes of considerable 

 biological importance on the physical side. Man 

 brings with him from his animal ancestors the endocrine 



