398 PALEONTOLOGY 



of their coming into being, no light has yet been thrown 

 by experiment or observation. As a cause of extinction 

 in times anterior to man, it is most reasonable to assign the 

 chief weight to those gradual changes in the conditions affect- 

 ing a due supply of sustenance to animals in a state of 

 nature which must have accompanied the slow alternations 

 of land and sea brought about in the reons of geological 

 time. Yet this reasoning is applicable only to land-animals ; 

 for it is scarcely conceivable that such operations can have 

 affected sea-fishes. There are characters in land-animals 

 rendering them more obnoxious to extirpating influences, 

 which may explain why so many of the larger species of par- 

 ticular groups have become extinct, whilst smaller species of 

 equal antiquity have survived. In proportion to its bulk is 

 the difficulty of the contest which, as a living organism, the 

 individual of such species has to maintain against the sur- 

 rounding agencies that are ever tending to dissolve the vital 

 bond, and subjugate the living matter to the ordinary chemical 

 and physical forces. Any changes, therefore, in such external 

 agencies as a species may have been originally adapted to exist 

 in, will militate against that existence in a degree proportionate 

 to the bulk of the species. If a dry season be gradually pro- 

 longed, the large Mammal will suffer from the drought sooner 

 than the small one ; if such alteration of climate affect the 

 quantity of vegetable food, the bulky Herbivore will first feel 

 the effects of stinted nourishment ; if new enemies be intro- 

 duced, the large and conspicuous animal will fall a prey, 

 while the smaller kinds conceal themselves and escape. Small 

 quadrupeds are more prolific than large ones. Those of the 

 bulk of the Mastodons, Megatheria, Glyptodons, and Diproto- 

 dons are uniparous. The actual presence, therefore, of small 

 species of animals in countries where larger species of the 

 same natural families formerly existed, is not the consequence 

 of degeneration — of any gradual diminution of the size of such 



