YOUTH IN BIRDS AND LOWER ANIMALS 57 



grow slowly, moulting five or six times in the first two years of their 

 life. In the seventeenth year they leave the ground, burrowing up 

 through the surface soil or through hard-trodden paths, and after 

 hiding for a time under stones and sticks, crawl up trees, where they 

 undergo the final moult, from which the perfect insect emerges. 



These various cases of the shortening of the adult life until it 

 leaves time only for reproduction must be secondary adaptations, 

 for it cannot be supposed that creatures with the elaborate structure 

 of winged insects could have come into existence without the 

 capacity to feed, and the extreme instances are connected by a 

 chain of intermediate forms with insects possessing a more normal 

 balance of the periods of life. Winged insects have many enemies ; 

 they are fed upon by all manner of reptiles, birds and mammals. 

 Weismann has suggested that the pressure of the struggle for 

 existence is so great that it has become of importance to them to 

 get through the business of reproduction as quickly as possible, and 

 that those insects have survived best and so have been favoured 

 by natural selection in which sexual maturity most quickly followed 

 the attainment of the adult form. In the extreme cases where the 

 insects became capable of reproduction immediately after their 

 final moult, and where little or no time had to be spent in choosing 

 suitable places for the eggs, it became unnecessary for the adults 

 to feed, and so their mouth-organs degenerated. This certainly 

 provides a reasonable explanation as to why the laying of eggs 

 should be hurried, for it is plain that the species would soon die 

 out if most of its adult members were killed off before they had 

 had time to lay the foundation of the next generation. It is a little 

 more difficult to understand, however, why the insects should die 

 so quickly, even although they have accomplished their task of 

 reproduction. Weismann suggested that this too was the result 

 of natural selection ; he supposed that it was an advantage to a 

 species to be represented by as many fresh and vigorous forms as 

 possible, and that just as a gardener removed faded flowers from 

 his floral borders and replaced them by younger and more vigorous 

 plants, so death came to weed out animals that had been battered 

 by the accidents of life, as quickly as possible after the maintenance 

 of the species had been secured by reproduction. He suggested, 

 further, that every animal was wound up, so to speak, only to 

 live for the time necessary to fulfil its cycle of life, and when 

 that had elapsed, the vital processes of repair and of removal 

 of wasted tissue which must continue to operate so long as an 



