WHY DO ANIMALS BECOME EXTINCT? 183 



winter quarters, and other long-extinct animals, too, 

 have been found under such conditions as to suggest a 

 similar fate. 



Among local catastrophes brought about by unusually 

 prolonged cold may be cited the decimation of the fur- 

 seal herds of the Pribilof Islands in 1834 and 1859, 

 when the breeding seals were prevented from landing 

 by the presence of ice-floes, and perished by thousands. 

 Peculiar interest is attached to this case, because the 

 restriction of the northern fur-seals to a few isolated, 

 long undiscovered islands, is believed to have been 

 brought about by their complete extermination in other 

 localities by prehistoric man. Had these two seasons 

 killed all the seals, it would have been a reversal of the 

 customary extermination by man of a species reduced 

 in numbers by nature. 



In the case of large animals another element probably 

 played a part. The larger the animal, the fewer young, 

 as a rule, does it bring forth at a birth, the longer are 

 the intervals between births, and the slower the growth 

 of the young. The loss of two or three broods of spar- 

 rows or two or three litters of rabbits makes com- 

 paratively little difference, as the loss is soon supplied, 

 but the death of the young of the larger and higher 

 mammals is a more serious matter. A factor that has 

 probably played an important role in the extinction of 

 animals is the relation that exists between various 

 animals, and the relations that also exist between 

 animals and plants, so that the existence of one is de- 

 pendent on that of another. Thus no group of living 

 beings, plants or animals, can be affected without in 

 some way affecting others, so that the injury or destrac- 

 tion of some plant may result in serious harm to some 

 animal. In this connection it has been suggested that 

 volcanic eruptions covering the earth for miles around 



