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 unaccountably 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 portions 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, sluggish 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 domesticated 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 exterminated 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 have taken place in the world's flora and fauna during recent times, his influence, as a whole, has been insignificant. Thus, while man exterminated the great northern sea-cow, Rytina, and Pallas's cormorant on the Commander Islands, these animals were already restricted to this circumscribed area[22] by natural causes, so that man but finished what nature had begun. The extermination of the great auk in European waters was somewhat similar. There is, however, this unfortunate difference between extermination wrought by man and that brought about by natural causes: the extermination of species by nature is ordinarily slow, and the place of one is taken by another, while the destruction wrought by man is rapid, and the gaps he creates remain unfilled. [22] _It is possible that the cormorant may always have been confined to this one spot, but this is probably not the case with the sea-cow._