WHY DO ANIMALS BECOME EXTINCT? 177 



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

 when the germ-plasm is incapable of further subdivi- 

 sion, 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 un- 

 accountably 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 another 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 that in some por- 

 tions of their range they may have been extirpated by a 

 change in food-supply, due in turn to a change in 

 climate, it seems preposterous to claim that there was 

 not at all times, somewhere in this vast expanse of 

 territory, a climate mild enough and a food-supply 

 large enough for the support of even these huge, slug- 

 gish creatures. We may evoke the aid of primitive man 

 to account for the disappearance of this race of giants, 

 and we know that the two were coeval in Patagonia, 

 where the sloths seem to have played the role of do- 

 mesticated animals, but again it seems incredible that 

 early man, with his flint-tipped spears and arrows, 

 should have been able to slay even such slow beasts as 

 these to the very last individual. 



Of course, in modern times man has directly extermi- 

 nated many animals, while by the introduction of dogs, 

 cats, pigs, and goats he has indirectly not only thinned 

 the ranks of animals, but destroyed plant life on an 

 enormous scale. But in the past man's capabilities for 

 harm were infinitely less than now, while of course the 

 greatest changes took place before man even existed, 

 so that, while he is responsible for the great changes that 



