68 METAMOEPHOSES OP MAN 



Thus we see that this apparatus, though it marks 

 out but an epoch in the lives of mammals and birds, 

 is here characteristic of an entire change in the mor- 

 phology of the animal. Its physiological significa- 

 tion is therefore, from- this circumstance alone, of far 

 greater value, and, as we shall see further on, this 

 value is sometimes enhanced. 



Here we must notice a very significant fact, and 

 one which bears forcibly on the present subject. The 

 female Pieris dies soon after depositing her eggs, 

 and the male ceases to exist at a still earlier period. 

 Matrimony is as fatal to them as it is to all insects, 

 and their existence terminates when the preservation 

 of the species has been insured. There is, however, 

 an occasional circumstance which, by preventing the 

 discharge of the functions essential to this final object, 

 prolongs the usually short life of these insects to an 

 extent beyond that which is intended. Some but- 

 terflies do not emerge from the chrysalis case till late 

 in autumn, and then the cold temperature of the sur- 

 rounding atmosphere retards their development, and 

 winter comes on ere they have begun their amours. 



In consequence of this, they retire to some sheltered 

 spot, remain there during the whole season, and 

 reappear in spring. Thanks to this condition of 

 celibacy, the result rather of circumstances than of 

 desire, their lives, instead of being limited to a few 

 weeks, extend over several months. 



