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 (Mus 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 i-at, 

 which the Maoris allege they brought mth 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 ^WU 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 ^-eater 

 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. 



