134 THE LIVING WORLD. 



Triassic (6), Jurassic (7), and Cretaceous (8) ages the 

 mammals were undergoing constant changes, and 

 were subject to numerous vicissitudes. Nothing 

 occurred, however, to give them any special ad- 

 vantage over their conditions or their enemies, and 

 they therefore continued for this long period with 

 little modification. But toward the end of the Cre- 

 taceous age it chanced that among the multitude of 

 variations some of the individuals acquired a new 

 character in regard to the habit of reproduction. 

 This new character gave them the power of retaining 

 their young in the uterus for a longer time than before, 

 carrying them indeed until they were well developed. 

 Now we know from the study of animals to-day that 

 this character, together with others correlated with it, 

 make the animal thus favored very much more pow- 

 erful and far better adapted to contend in the strug- 

 gle for life. We may not be able to say why it is, 

 but it is certainly true that the mammals with this 

 new habit in reproduction always triumph over the 

 marsupials. It is easy to suppose, therefore, that 

 as soon as this variation occurred in the mammalian 

 stock there was an immediate impulse given to the 

 development of the group. The mammals became 

 more active, and soon proved themselves more than 

 a match for the large reptiles. In the struggle for 

 food they soon triumphed over their marsupial pre- 

 decessors, and finding thus the whole world open to 

 their conquests, they multiplied rapidly and gave 

 rise at once to the multiplicity of types found in the 

 early Tertiary (9). A new field had thus been offered 

 by a chance (?) anatomical variation. 



