LIFE OF TERTIARY PERIOD 203 



single young one every three years, while a rahbit may have a litter 

 of seven or eight young two or three times a year. Now the prob- 

 ability of useful variations will be in direct proportion to the popu- 

 lation of the species, and, as the smaller animals are not only many 

 hundred times more numerous than the largest, but also increase 

 perhaps a hundred times as rapidly, they are able to become quickly 

 modified by variation and natural selection, while the large and 

 bulky species, being unable to vary quickly enough, are obliged to 

 succumb in the struggle for existence/^ 



To these reasons we may add that very large animals arc 

 less rapid in their motions, and thus less able to escape from 

 enemies or from many kinds of danger. The late Professor 

 O. Marsh, of Yale University, has well observed : 



" In every vigorous primitive type which was destined to survive 

 many geological changes, there seems to have been a tendency to 

 throw off lateral branches, which became highly specialised, and 

 soon died out because they were unable to adapt themselves to new 

 conditions. . . . The whole narrow path of the Suilline (hog) 

 type, throughout the entire series of the American tertiaries, is 

 strewn w4th the remains of such ambitious offshoots, manv of them 

 attaining the size of a rhinoceros; while the typical pig, with an 

 obstinacy never lost, has held on in spite of catastrophes and evolu- 

 tion, and still lives in America to-day." 



We may also remember that it is still more widely spread 

 over the Old World, under the various forms of the hojr-family 

 (Suidse), than it is in America, under the closely allied 

 peccary type (Dicotylida?). 



That this is a true cause of the more frequent passing away 

 of the largest animal types in all geological epochs there can 

 be no doubt, but it certainly will not alone explain the dying 

 out of so many of the very largest ^Mammalia and birds dur- 

 ing a period of such limited duration as is the Pleistocene 

 (or Quaternary) age, and under conditions which were cer- 

 tainly not very different from those under whicli they had 

 been developed and had lived in many cases down to the 

 historical period. 



