WHY DO ANIMALS BECOME EXTINCT? 233 
suger, was killed off in the West 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 
