158 EXTINCT ANIMALS OF AUSTRALIA. [part ii. 



tion could have occurred in a country like Australia ; but if the 

 ocean sank 2,000 feet, the great eastern mountain range might 

 have given rise to local glaciers. It is, however, almost certain 

 that during late Tertiary times Australia must have been much 

 more extensive than it is now. This is necessary to allow of the 

 development of its peculiar and extensive fauna, especially as we 

 see that that fauna comprised animals rivalling in bulk those of 

 the great continents. It is further indicated by the relations 

 with New Guinea, already alluded to, and by the general character 

 of the various faunas which compose the Australian region, de- 

 tails of which will be found in the succeeding part of this work. 

 The lowering of the ocean during the Glacial period would be 

 favourable to the still further development of the fauna of such 

 a country ; and it is to the unfavourable conditions produced by 

 its subsequent rising — equivalent to a depression of the land to 

 the amount of two thousand feet — that we must impute the 

 extinction of so many remarkable groups of animals. It is not 

 improbable, that the disappearance of the ice and the consequent 

 (apparent) subsidence of the land, might have been rapid as 

 compared with the rate at which large animals can become 

 modified to meet new conditions. Extensive tracts of fertile 

 land might have been submerged, and the consequent crowding 

 of large numbers of species and individuals on limited areas 

 would have led to a struggle for existence in which the less 

 adapted and less easily modifiable, not the physically weaker, 

 would succumb. 



There is, however, another cause for the extinction of large 

 rather than small animals whenever an important change of 

 conditions occurs, which has been suggested to me by a corre- 

 spondent, 1 but which has not, I believe, been adduced by Mr. 

 Darwin or by any other writer on the subject. It is dependent 

 on the fact, that large animals as compared with small ones are 

 almost invariably slow breeders, and as they also necessarily 

 exist in much smaller numbers in a given area, they offer far 

 less materials for favourable variations than do smaller animals. 

 In such an extreme case as that of the rabbit and elephant, the 



1 Mr. John Hiekman of Desborongh. 



