NATURAL SELECTION 



21 



in these cases it is easily seen that the life of the parent 

 will naturally be continued until the completion of the 

 period of parental care over the offspring. In the case of 

 animals which form communities, it may be advantageous 

 to these communities to have their members continue to live 

 even after their reproductive activity ceases, since they may 

 aid the community in other ways than by reproduction. 



Let us see a 

 few concrete il- 

 lustrations of this 

 principle that in 

 the processes of 

 natural selection 



the welfare of the fl g^ T ^^ * 

 race and not of 

 the -individual is 

 sought. Very 

 commonly seen 

 on our trees are 

 the egg-cases of 

 the bag-worm 

 (Fig. 3), a moth, 

 the female of which never comes to complete development, 

 in fact, never leaves the cocoon, but is fertilized by the male 

 and lays her eggs without ever emerging into a free life as 

 an active, flying adult. More than this, not only is the 

 active life of the adult female suppressed : her body disin- 

 tegrates in the process of laying the eggs, so that ovulation 

 and the death of the female are simultaneous. Here we 

 see the continued existence of the adult female after the 

 eggs are laid is of no value to the species, and she is 



FlG. 3. The "bag-worm," Thyroidopteryx ephemeriformis. 



a. Larva, b. Pupa. c. Adult female (wingless), d. Adult male. 

 e. Longitudinal section of a cocoon showing the degenerate female 

 full of eggs. f. One of the larvae, showing the covering of silk and 

 twigs in which the posterior part of the body is enclosed, g. Young 

 larvae, natural size. By the courtesy of the United States Depart- 

 ment of Agriculture. 



