u8 



ZOOLOGY 



cocial). The young of the song birds, as the thrushes, swallows, 

 etc., are wholly dependent on the care of the parents for a con- 

 siderable time (altricial). In the herbivorous mammals, as the 

 sheep and cattle, the young have the use of their limbs in a short 

 time after birth. Among the carnivorous forms, as the cat and 

 dog, the young are more helpless. In the human species the 

 period of helplessness is longest and consequently the necessity 



FIG. 56. 



FIG. 56. Nestling Marsh Hawks (Circus cyaneus). From Year-Book. Department of 



Agriculture. 



Questions on the figure. What are the nesting and breeding habits of the 

 marsh hawk? Are the young precocial or altricial? 



of parental care greatest. In general, the longer period of 

 parental protection accompanies the development of more com- 

 plex and highly organized instincts, and intelligence. The 

 lengthened period of dependence, while a burden to the parent 

 in one sense, is an advantage to it in the saving in number of 

 offspring, and serves to benefit the species, not merely by keep- 

 ing the offspring alive until they may reproduce, but in the 

 greater development of such parental instincts as gentleness, 

 self-sacrifice, and the like. In the human race it has given rise 



