METAMORPHOSIS. 217 



stimuli of development. Suppose, further, that the 

 new conditions of existence while remaining for a 

 long time the same for each individual, should dif- 

 fer slightly for successive generations. The effect 

 would be to hinder still further the action of the 

 hereditary impulse. The total effect, therefore, of 

 these new conditions would be to prolong the cater- 

 pillar state, — there would be a tendency to remain 

 in this state. But the hereditary impulse is not 

 destroyed, and the mature development of the insect 

 eventually occurs, the only change being the extreme 

 prolongation of the caterpillar stage. Suppose now 

 some change of climatic conditions, whereby as each 

 generation is about to abandon its protracted cater- 

 pillar stage, it encounters cold weather. Each cater- 

 pillar protects itself with a wrapping of leaves or a 

 web of its own spinning ; and because of the strong 

 associations of hereditary impulse, development pro- 

 ceeds — a supply of energy from the previous abun- 

 dance of nutriment being on hand. In this quiet, 

 motionless, and protected state, the conditions of 

 development are the same for each individual and 

 each successive generation, therefore the hereditary 

 impulse for this period thus becomes invariable in its 

 results, more rapid, and its various stages become 

 compressed. The last changes of development, as is 

 universal in the organic world, tend to occur earlier 



