194 DEVELOPMENT AND HEREDITY. 



brood develops under circumstances which are 

 different from those of the other broods. The dif- 

 ference lies chiefly in the amount of nourishment 

 supplied to each brood. These varying conditions 

 occur every year, and always in each year in the 

 same order, thus forming regular cycles in the 

 life of each species. We must recognise the cause 

 of the three unlike broods of females in the three 

 conditions of food-supply. While the species has 

 been advancing in its development to the produc- 

 tion of the new and more perfect forms of the large 

 queens, the regularly recurring period of the meagre 

 food-supply has caused each year the reproduc- 

 tion of the imperfect ancestral form — the stunted 

 worker. 



The honey-bee is a species which has developed 

 further along the same lines. Its forms are more 

 specialised than those of the bumblebee, and seem 

 to be more differentiated from one another. But 

 the difference is not so great that intermediate 

 forms may not occasionally occur. 



In the case of the butterflies we have seen that 

 the change of the seasons was the cause of the 

 existence of two or more forms in a species, but 

 we have previously seen that heat and cold are 

 not the only conditions which advance or retard 

 development. Light is almost as important, and 



