34 
DARWINISM 
CHAP. 
meant, the missel-thrush has increased in numbers in Scotland 
during the last thirty years, and this has caused a decrease in 
the numbers of the closely allied song-thrush in the same 
country. The black rat (Mils rattus) was the common rat of 
Europe till, in the beginning of the eighteenth century, the 
large brown rat (Mus decumanus) appeared on the Lower 
Volga, and thence spread more or less rapidly till it overran all 
Europe, and generally drove out the black rat, which in most 
parts is now comparatively rare or quite extinct. This invad¬ 
ing rat has now been carried by commerce all over the world, 
and in New Zealand has completely extirpated a native rat, 
which the Maoris allege they brought with them from their 
home in the Pacific ; and in the same country a native fly is 
being supplanted by the European house-fly. In Russia the 
small Asiatic cockroach has driven away a larger native species; 
and in Australia the imported hive-bee is exterminating the 
small stingless native bee. 
The reason why this kind of struggle goes on is apparent 
if we consider that the allied species fill nearly the same place 
in the economy of nature. They require nearly the same 
kind of food, are exposed to the same enemies and the same 
dangers. Hence, if one has ever so slight an advantage over 
the other in procuring food or in avoiding danger, in its 
rapidity of multiplication or its tenacity of life, it will increase 
more rapidly, and by that very fact will cause the other to 
decrease and often become altogether extinct. In some cases, 
no doubt, there is actual war between the two, the stronger 
killing the weaker; but this is by no means necessary, and 
there may be cases in which the weaker species, physically, 
may prevail, by its power of more rapid multiplication, its 
better withstanding vicissitudes of climates, or its greater 
cunning in escaping the attacks of the common enemies. 
The same principle is seen at work in the fact that certain 
mountain varieties of sheep will starve out other mountain 
varieties, so that the two cannot be kept together. In plants 
the same thing occurs. If several distinct varieties of wheat 
are sown together, and the mixed seed resown, some of the 
varieties which best suit the soil and climate, or are naturally 
the most fertile, will beat the others and so yield more seed, and 
will consequently in a few years supplant the other varieties. 
