64 MAMMALIAX DESCENT. [Lect. III. 



it were one body witli many members, may justly be 

 called earth -life. 



In this group — the Marsuj^ialia — the general intelli- 

 gence is low, the brain has but few convolutions, and 

 the great familiar jjridge, over which nerve-impressions 

 travel right and left — the corpus callosum of our 

 brain — is thin and feeble in these types. At first 

 sight these Metatheria appear to be a very neat group, 

 a peo23le quiet and secure, having no business with 

 any other tribes, but living in their own zoological 

 seclusion. But, as in all similar cases, this is only 

 a23parent ; the hedge set about them, and all that they 

 have, is as unsubstantial as a dream. Yet they are a 

 feeble folk, and their structure, habits, and distribution 

 in space and in time are all congruent to this view of 

 the j^ouched tribes. 



The noble beasts, like the nobler tribes of men, are 

 " mighty hunters," and they have driven the feebler 

 MarsujDial tribes before them. Hence it is that in these 

 days, " Wallace's line " bounds them on the north, in 

 the Eastern world ; while in the Western continent, only 

 one genus (Didelphys, or the Opossums) lingers amongst 

 the Eutherian types, and one or two sj^ecies have found 

 their way over that great western icorld-link, the 

 Isthmus of Panama, yet the real home of that genus is 

 in the southern, and not properly in the northern 

 half of the American Continent at all. 



But the loss of so many of these low types, in this 



