262 THE WORLD OF LIFE 



which iindo"abtedly survived that event have since become ex- 

 tinct. This great climatic catastrophe did nndoubtedly pro- 

 duce extensive migration of Mammalia ; but, owing to the fact 

 that the ice-sheet had very definite limits, and that numbers 

 of large mammals were merely driven southward, it is not 

 held to be a sufficient cause for so general a destruction of the 

 larger forms of life. 



Another circumstance that puts the glacial epoch out of 

 court as a sufficient explanation of the widespread extinction 

 is that in two very remote parts of the earth, both enjoying 

 a warm or even a sub-tropical climate — x\ustralia on the one 

 hand, and Brazil to Argentina on the other, — exactly the 

 same phenomena have occurred, and, so far as all the geo- 

 logical evidence shows, within the same general limits of time. 



It is no doubt the case that at each of the dividing lines 

 of the Tertiary era — that is, in passing from the Eocene to 

 the Miocene, or from the latter to the Pliocene, and thence 

 to the Pleistocene — many large Mammalia have also become 

 extinct. But in these cases a much greater lapse of time can 

 be assumed, as well as larger changes in the physical condi- 

 tions, such as extension of land or water, climate, vegetation, 

 etc., which, combined with the special disabilities of very 

 large animals, are sufficient to account for the facts. It may 

 be well here to state again the causes which lead to the ex- 

 tinction of largo 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 impor- 

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

 slowly as compared with small ones — the elephant producing a 



