WHY DO ANIMALS BECOME EXTINCT? 233 



siger, was killed off in the ^'N^est by blizzards 

 when the animals were gathered in their win- 

 ter 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 Prib- 

 ilof Islands in 1834 and 1859, when the breed- 

 ing seals were prevented from landing by the 

 presence of ice-floes, and perished by thou- 

 sands. 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 lo- 

 calities 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 ani- 

 mal, 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 



