394 DARWINISM chap. 



feet and teeth, as in the American fossil horses; or in the in- 

 creased development of the branching horns, as in the true 

 deer. In each of these cases specialisation and adaptation to 

 the conditions of the environment appear to have reached their 

 limits, and any change of these conditions, especially if it be 

 at all rapid or accompanied by the competition of less developed 

 but more adaptable forms, is liable to cause the extinction of 

 the most highly developed groups. Such we know was the 

 case with the horse tribe in America, which totally disappeared 

 in that continent at an epoch so recent that we cannot be 

 sure that the disappearance was not witnessed, perhaps caused, 

 by man; while even in the Eastern hemisphere it is the 

 smaller species — the asses and the zebras — that have persisted, 

 while the larger and more highly developed true horses have 

 almost, if not quite, disappeared in a state of nature. So we 

 find, both in Australia and South America, that in a quite 

 recent period many of the largest and most specialised forms 

 have become extinct, while only the smaller types have sur- 

 vived to our day ; and a similar fact is to be observed in many 

 of the earlier geological epochs, a group progressing and reach- 

 ing a maximum of size or complexity and then dying out, 

 or leaving at most but few and pigmy representatives. 



Cause of Extinction of Large Animals. 



Now there are several reasons for the repeated extinction 

 of large rather than of small animals. 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 smaller animals. In the next place, 

 the extreme specialisation of many of these large animals 

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

 direction suited to 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 favourable 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 



