WHY DO ANIMALS BECOME EXTINCT? 223 



not have found these creatures still disporting 

 themselves in the Gulf of Mexico. The best 

 we can do is to fall back on an unknown " law 

 of progress," and say that the trend of life is 

 toward the replacement of large, lower animals 

 by those smaller and intellectually higher. 



But why there should be an allotted course 

 to any group of animals, why some species 

 come to an end when they are seemingly as 

 well fitted to endure as others now living, we 

 do not know, and if we say that a time comes 

 when the germ-plasm is incapable of further 

 subdivision, we merely express our ignorance 

 in an unnecessary number of words. The 

 mammoth and mastodon have already been 

 cited as instances of animals that have unac- 

 countably become extinct, and these examples 

 are chosen from among many on account of 

 their striking nature. The great ground sloths, 

 the Mylodons, Megatheres, and their allies, are 

 another case in point. At one period or an- 

 other they reached from Oregon to Virginia, 

 Florida, and Patagonia, though it is not 

 claimed that they covered all this area at one 

 time. And, while it may be freely admitted 



