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,! 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 Hickman of Desborough. 
