io7 
of Europe the black rat is even the more numerous ; in 
England the black rat flourished for five centuries before 
the advent of the brown rat. The English watercress in 
New Zealand was certainly adapted most perfectly to the 
conditions in which it was placed, for it grows “ so luxuri 
antly in that country as to completely choke up the rivers, 
sometimes leading to disastrous floods, and necessitating 
great outlay to keep the stream open ; and yet it has 
been exterminated from the very localities in which it 
flourished.” 
In these cases there is no mystery in the victory of one 
species over the other. We may explain the extermination 
of the black rat by the brown rat, from England, by 
supposing that there were special circumstances which had 
not occurred elsewhere. It appears that, in the countries 
where both kinds of rats are to be found, their habits and 
their haunts are different, and hence they do not come into 
collision with one another. But if, in consequence of 
coming over in ships, the brown rat acquired the habit of 
picking up a living in places where man dwelt, it might 
then come into immediate contact with the native settlers. 
We may further suppose that prior to the arrival of the 
brown rat, the black rat occupied all the ground which could 
be occupied by rats of their habits of life, in consequence 
of the presence of enemies or through the limited supply of 
food. The invader comes into competition with this long 
naturalised cousin of his. Both species tend to increase 
at an enormous rate, but there is not room for both. And 
then begins the competition between race and race. Now 
the invader was the stronger and the larger of the two 
species ; it had also the fiercer disposition, and there does 
not seem to be much mystery in the fact that it gained the 
ascendancy. There are many ways in which this might 
be brought about. The most obvious way would be for