MAMMALS 335 



young alive. We found exceptions in the duckbill and 

 the spiny ant-eater, both of which lay eggs. 



Upward progression among the mammals. — The duck- 

 bill, the lowest mammal, ushers in the class. Superficially, 

 this animal resembles a bird so that one might think the 

 Mammalia more closely related to the Aves than to any 

 other class among the vertebrates. Modern zoologists are, 

 however, inclined to think the mammals had for an ancestor 

 some animal of very strong reptilian characters, because 

 the duckbill and spiny ant-eater possess reptilelike charac- 

 ters. Whatever kind of an ancestor mammals had, they 

 show a great advance over any other group of animals in 

 complexity of structure. A most important advance among 

 the mammals themselves is a constant tendency for the 

 body to develop headward. The brain becomes larger 

 and larger, and the fore limbs become modified into hands 

 for grasping, exemplified in man. Moreover, there is a 

 development among the mammals toward an upright 

 position, again exemplified in man. This " holding up of 

 the head " seems to go along with the increase in size of 

 brain. The complexity of the teeth and the structure of 

 the limbs show a decided step in advance over the birds and 

 reptiles. 



Adaptations to environments among the mammals. — 

 Whatever forces compelled the duckbill to live the life it 

 does, other forces just as surely molded and adapted it to 

 that life. Its food is found mainly under water, and to 

 obtain this food, the duckbill is obliged to remain sub- 

 merged some time. Consequently, although a terrestrial 

 animal, it has become adapted to staying beneath the water 

 some six or seven minutes at a time. Its feet are webbed 

 for swimming, yet the web on the front pair can be turned 



