146 THE LIVING WORLD. 



dency toward convolution. There is hardly a more 

 significant fact in the history of animals, when we 

 remember the approaching advent of man, than this 

 increase in the size of the brain of mammals from 

 the early Tertiary (9). Undoubtedly this increase 

 in the size of the brain was connected with the in- 

 creased activity of the mammals already mentioned, 

 for activity and agility imply delicate control of the 

 lower motor centres by the higher centres of the 

 brain, and this requires more brain power. Active 

 animals always have relatively large brains. But 

 undoubtedly also the increase in the size of the 

 brain was accompanied by an increasing amount of 

 intelligence. In short, with the development of the 

 mammals there had appeared a time when bulk had 

 given place to muscular activity and agility, but at 

 the same time the growth of the brain was gradually 

 preparing the way for the time when intelligence 

 should take the place of brute force. 





 Instinct. 



Already, in an entirely different line of descent, do 

 we find the mental nature becoming predominant. 

 The higher order of insects has as its chief 

 character the habit of living in colonies, and the 

 consequent development of remarkable instincts. 

 As a rule, the insects seem to have depended upon 

 their powers of rapid multiplication as their chief 

 means of defence. Most insects are weak animals, 

 and all are small, but their immense powers of re- 

 production safely defend them from extermination. 

 Some of the higher orders, however, learned to band 



