XII 



LIFE OF TERTIARY PERIOD 245 



very large animals, are sufficient to account for the facts. 

 It may be well here to state again the causes which lead 

 to the extinction of large animals rather than small ones, as 

 given in my Darwinism (p. 394) more than twenty years 

 ago, and also in my Geographical Distribution of Animals, 

 i. p. 157 (1876): 



" In the first place, animals of great bulk require a proportionate 

 supply of food, and any adverse change of conditions would affect 

 them more seriously than it would affect smaller animals. In the 

 next place, the extreme specialisation of many of these large animals 

 would render it less easy for them to become modified in any new 

 direction required by the changed conditions. Still more important, 

 perhaps, is the fact that very large animals always increase slowly 

 as compared with small ones — the elephant producing a single young 

 one every three years, while a rabbit may have a litter of seven 

 or eight young two or three times a year. Now the probability of 

 useful variations will be in direct proportion to the population 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 

 are 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 

 with the remains of such ambitious offshoots, many 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 evolution, 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 hog- 

 family (Suidae), than it is in America, under the closely 

 allied peccary type (Dicotylidae). 



