319 
“The exact causes of this displacement (of one set of species 
by another) are by no means of such a simple nature. All the plants 
concerned may be perfectly hardy, all may grow freely from seed, yet 
when left alone for a number of years each set is in turn driven out 
by a succeeding set, till at the end of a considerable period—a 
century or a few centuries, perhaps—hardly one of the plants which 
first monopolised the ground would be found there.”—(Wallace. 
Darwinism, p. 15.) 
It is possible that a group of variants which arise in 
connection with any species will compete with the parent 
form ; and that form which is best adapted to the con 
ditions will survive. 
Speaking of the reasons which lead especially to the 
extinction of large animals, Mr. Wallace says :— 
“ There is, however, another cause for the extinction of large 
rather than small animals whenever an important change of con 
ditions occurs, which has been suggested to me by a correspondent, 
but which has not, I believe, been adduced by Mr. Darwin nor by 
any other writer on this 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 young born each year in the 
world are probably as some millions to one ; and it is very easily 
conceivable that in a thousand years the former might, under pres 
sure of rapidly changing conditions, become modified into a distinct 
species, while the latter, not offering enough favourable variations to 
effect a suitable adaptation, would become extinct.”—(Wallace. The 
Geographical Distribution of Animals, vol. /., pp. 158-9.) 
Two other somewhat exceptional causes remain to be 
indicated. Mr. Wallace says :— 
“ It is clear that we are now in an altogether exceptional period of 
the earth’s history. We live in a zoologically impoverished world, 
from which all the hugest and fiercest and strangest forms have 
recently disappeared ; and it is no doubt a much better world for us 
now they have gone. . . . We cannot but believe that there must 
have been some physical cause for this great change ; and it must 
have been a cause capable of acting almost simultaneously over 
large portions of the earth’s surface, and one which as far as the