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 
