46o BIOLOGY AND HUMAN LIFE 



There is a striking differentiation between the anterior and 

 posterior appendages (wings and legs) (see Fig. i88) ; and 

 there are differentiated forms of beaks as well as of legs (see 

 Fig. 189). The eye is very well developed, as is the sense of 

 hearing. The sense of smell, while present, is not particularly 

 keen. Related to the care of eggs and young are many special- 

 ized instincts connected with nest-building and food-gathering. 



The poultry and eggs produced in this country and sold for 

 food every year are valued at over $500,000,000. To this must 

 be added the quantities consumed by the people who raise them, 

 the imported bird products, and the game birds used as food, 

 the value of which it is practically impossible to estimate. Im- 

 ported feathers and downs come to about $8,000,000 a year. 



The most valuable organic fertilizer consists of guano, which 

 is the refuse of millions of birds, accumulated through a long 

 stretch of years on various islands off the coast of South 

 America. 



Of all animals the common birds yield most pleasure to man, 

 with their song and chatter, their beautiful appearance, and 

 their interesting activities ; but the greatest importance of birds 

 to man is found to lie in their relation to insects and weeds. 

 They are thus significant not so much by what they produce for 

 us, even if we make full allowance for the food and feathers and 

 fertilizer that they yield, as by the help they give us in conserv- 

 ing or protecting our crops (see Chapter L). 



340. Mammals. The name mammal comes from a word mean- 

 ing breast and refers to the milk glands by means of which 

 animals of this division nourish the newborn young. The class 

 is also characterized by a skin covered with hair, and by a 

 separation of the body cavity into thorax and abdomen by the 

 diaphragm (see Fig. 124). 



Our most important protein foods come from the hoofed ani- 

 mals — not only meat but milk and its derivatives and modifica- 

 tions. The most useful single animal in relation to our clothing 

 is the sheep. The most useful animal in relation to work and 

 transportation is the horse. Our furs are all mammalian prod- 



