72 THE WHENCE AND THE WHITHER OF MAN 



of small size having an external skeleton. Whatever 

 may be our explanation the fact remains that insects 

 are always small. This is in itself a disadvantage. 

 Yery small animals cannot keep up a constant high 

 temperature unless the surrounding air is warm, for 

 their radiating surface is too large in comparison with 

 their heat-producing mass. At the first approach of 

 even cool weather they become chilled and sluggish, 

 and must hibernate or die. They are conformed to 

 but a limited range of environment in temperature. 



But small size is, as a rule, accompanied by an even 

 greater disadvantage. It seems to be almost always 

 correlated with short life. Why this is so, or how, we 

 do not know. There are exceptions ; a crow lives as 

 long as a man ; or would, if allowed to. But, as a rule, 

 the length of an animal's days is roughly proportional 

 to the size of its body. And the insect is, as a rule, 

 very short-lived. It lives for a few days or weeks, or 

 even months, but rarely outlasts the year. It has 

 time to learn but little by experience. The same ex- 

 perience must be passed, the same emergency arise and 

 be met, over and over again during the lifetime of the 

 same individual if the animal is to learn thereby. 

 And intelligence is based upon experience. Hence in- 

 sects can and do possess but a low grade of intelligence. 

 But instinct is in many cases habit fixed by heredity 

 and improved by selection. The rapid recurrence of 

 successive generations was exceedingly favorable to 

 the development of instincts, but very unfavorable to 

 intelligence. Insects are instinctive, the highest 

 vertebrates intelligent. The future can never belong 

 to a tiny animal governed by instincts. Mollusks and 

 insects have both failed to reach the goal ; another 



