52 
great numbers in all other parts of America where open pastures 
afforded suitable conditions. Asses, about fifty years after their 
introduction, ran wild and multiplied so amazingly in Quito, that 
the Spanish traveller Ulloa describes them as being a nuisance. 
They grazed together in great herds. . . . Hogs were turned 
out in St. Domingo by Columbus in 1493, and the Spaniards took 
them to other places where they settled, the result being that in 
about half a century these animals were found in great numbers 
over a large part of America, from 25 0 north to 40° south latitude. 
More recently, in New Zealand, pigs have multiplied so greatly 
in a wild state as to be a serious nuisance and injury to agriculture. 
To give some idea of their numbers, it is stated that in the province 
of Nelson there were killed in twenty months 25,000 wild pigs.”— 
(Wallace. Darwinism, pp. 27-28.) 
“ Several of the plants, such as the cardoon and a tall thistle, 
which are now the commonest over the wide plains of La Plata, 
clothing square leagues of surface almost to the exclusion of every 
other plant, have been introduced from Europe.”—(Origin of Species. 
P- 5i-) 
In the cases which have just been cited the principal 
reasons for the increase, no doubt, are—a not too largely- 
stocked country, plenty of food, a suitable climate, and 
absence of enemies. It is obvious in these cases that 
external conditions are favourable to the race, and 
that it could gain no advantage by being modified, and 
that therefore the transformation of species by Natural 
Selection cannot take place in this connection. 
But sometimes, without any transportation to another 
country, there comes a great “wave of life.” Circumstances 
suddenly become favourable to some quickly breeding 
species, and even in the course of one season the earth 
seems to teem with them. In this case also it is 
obvious that so long as there is a realised tendency 
to increase in a geometrical ratio, nothing like Natural 
Selection can take place. But the period during which 
this “ wave of life ” continues to flow in is generally 
limited, and we might almost suppose that there were 
laws of nature at work to arrest the abnormal fertility.