128 ESSAYS OF A BIOLOGIST 



in the two groups, we see the evolution not only 

 of certain structural characters such as division of 

 heart, compactness of skeleton, increase of brain-size, 

 not only of physiological characters like warm-blooded- 

 ness or efficiency of circulation, but also of various 

 psychical characters. The power of profiting by 

 experience becomes greater, as does that of distinguish- 

 ing between objects ; and there is most markedly 

 an increase in the intensity of emotion. It has 

 somehow been of advantage, direct or indirect, to 

 birds to acquire a greater capacity for affection, for 

 jealousy, for joy, for fear, for curiosity. In birds 

 the advance on the intellectual side has been less, on 

 the emotional side greater : so that we can study 

 in them a part of the single stream of life where 

 emotion, untrammelled by much reason, has the 

 upper hand. 



