PROGRESS OF THE BACKBONED FAMILY. 



747 



tropics, and even in colder countries glide rapidly along in the warm 

 sunshine, or hide in nooks and crannies, and sleep the winter away ; 

 and the birds the merry, active, warm-hearted birds live every- 

 where. 



Yet still the great backboned division is not exhausted ; on the 

 contrary, the most powerful if not the most numerous group is still to 



Fig. 1. 



come the mammalia, or milk-giving animals. Let us first notice two 

 important changes which give them an advantage over other back- 

 boned creatures. We have found the fish casting their eggs out into 

 the water, and, as a rule, taking no more thought of them ; so it was 

 again with the frogs, so with the reptiles, whose eggs, even when care- 

 fully buried by the mother, are often devoured by thousands before 

 the little ones have a chance of creeping out of the shell. And with 

 the birds, in spite of the parents' care, more eggs probably are eaten 

 by snakes, weasels, field-rats, and other creatures, than remain to be 

 hatched. 



Now, the cat and the cow, as we all know, do not lay eggs as birds 

 do ; but the mother carries the young within her body till they are 

 born, perfectly formed, into the world. And when at last her little 



