THE PERPETUATION OF LIFE 



339 



The young which are hatched in a weak and helpless condi- 

 tion are fed, brooded and protected by their parents who 

 usually keep the nest clean and often continue to care for 

 their young after they have left the nest and are appar- 

 ently quite able to secure food for themselves. Among 

 primitive types which produce a large number of offspring 

 there is a great waste of life. A codfish may lay over 

 9,000,000 eggs of which as a rule only two produce fishes 



FIG. 231. Bluebird at edge of nest with grasshopper in mouth for 

 young. (After Baker.) 



that live to a mature age. A humming bird which care- 

 fully rears its young in a well-prepared nest lays but two 

 eggs in a season. With parental care a species no longer 

 needs the enormous fecundity of the primitive forms which 

 leave their eggs and young to the mercies of the elements 

 and numerous enemies. 



In the mammals parental care is universal. The young 

 are closely dependent on their parents for food which is 

 supplied by the mammary glands and the instinct to 

 suckle and protect the young is a part of the endowment 



