INFANCY AMONG MAMMALS 



299 



immediately. Among other birds, such as robins and eagles, the 

 young birds are quite helpless. The feathers have not developed, 

 the eyes are closed, and the nestlings are not able to feed them- 

 selves. The parents fly about gathering insects, fish, lambs, or 

 whatever it is that constitutes their food. They bring this food 

 home to the nestlings. They protect them from other animals 

 and teach them, in due time, how to fly and get food for them- 

 selves. Only when the young are strong enough to fly and forage 

 for themselves are they put out of the nest. Birds lay compara- 

 tively few eggs because, due to internal fertilization and parental 

 care, each egg produced is almost sure to develop into a young 

 animal. Birds usually lay one egg at a time, and when the number 

 that is usual for the particular variety 

 of bird is reached, the parents start 

 to incubate the eggs. Compare this 

 number, one to sixteen, with the mil- 

 lions of eggs produced by the salmon 

 and the lack of parental care after 

 the salmon eggs are laid and sprayed 

 with milt. 



Infancy among mammals. The 

 embryo of a mammal is protected, by 

 the mother's body, a longer time than 

 that of any other animal. During 

 this period, development goes on, 

 and at birth, the young is easily rec- 

 ognizable as belonging to a particular 

 species. The embryos of different 

 species take varying lengths of time 

 to develop. The rabbit develops 

 from the fertile egg in three weeks and 

 then is born ; the human baby needs about nine months ; and the 

 elephant about two years. This shows remarkably rapid develop- 



Care of young is often shown by 

 birds. The tailor bird weaves an elab- 

 orate hanging nest in which the eggs 

 are hatched and the young cared for. 

 Both the mother and father birds brood 

 over them. One forages for food while 

 the other keeps the nestlings warm. 



