^^^^ ZOOGENESIS "^^^^ 



not possible in any other group of land living 

 vertebrates. 



This relative independence of their surroundings re- 

 sulting from the extreme keenness of their sight, 

 coupled v^ith the capacity for very rapid locomotion, 

 is correlated v^ith a definite dependence on certain 

 physical conditions v^hich is by no means so strongly 

 marked in any other creatures. 



Sight being all-important for the birds, the relative 

 length of day and night becomes an important factor 

 in their lives. For some birds, particularly in the 

 southern hemisphere v^here the seasons are not so 

 strongly marked as in the north, this probably has an 

 important bearing on migrations, since a longer day 

 means a greater proportion of the time w^hen sight is 

 an aid in securing food and in avoiding enemies. 



Such migrations as those of the golden plover and 

 of the Arctic tern, v^hich annually pass from far 

 northern to far southern regions and back again, may 

 perhaps largely be explained as an endeavor always 

 to keep w^ithin the longest possible day. 



While in the great majority of birds their interrela- 

 tionships v^ith other living things and with the other 

 objects in the world about them are governed mainly 

 by the sense of sight, all birds have wonderfully good 

 ears. For instance we notice the large thrush that 

 we in America call the robin listening at an earth- 

 worm burrow to find out whether or not the prospec- 

 tive victim is at home. It is probably listening for 

 the high pitched sounds made by the small chitinous 

 hooks on the earthworm's body VN^hich in small ones 



