EVOLUTION 1055 



ness. When the Brown Rat {Rattus norvegicus) found its way to 

 Britain in the early eighteenth century, it proved itself hardier, 

 more plastic, and more fecund than the Black Rat (R. ralius), 

 with the result that in fifty years the latter was almost exterminated 

 except in places where it was continually being re-introduced by 

 ships. But while the direct competition was real, it cannot be for- 

 gotten that man redoubled his e:fforts when the second rat appeared 

 on the scene, and that such changes as the replacement of wooden 

 houses by brick and stone, and the introduction of sewers, favoured 

 the stronger species. 



(3) Sometimes the influence is more subtle. Thus an intrusion of 

 rabbits into a particular area may reduce the number of hares, 

 simply because the two species are not on friendly terms, and 

 because the hares are repelled when plants have been contaminated 

 by rabbits. An extension of squirrels into an area may be followed 

 by the reduction of the number of wood-pigeons. There does not 

 seem at first sight any intersection of the two lives; but squirrels, 

 vegetarian as they essentially are, cannot resist killing and eating 

 the young squabs in the nest; and this is, from the farmer's point 

 of view, a useful check. 



(4) The introduction of an animal into a new country may involve 

 the introduction of its parasites, thus rats harbour rat-fleas, which 

 disseminate bubonic plague ; and it is also from rats that pigs, and 

 thence men, become infected with the disease of trichinosis, which 

 is due to a small Nematode (Trichinella spiralis). The re-introduction 

 of mosquitoes into an area which had been free from them for years 

 may be followed by a recrudescence of malaria. Many of these inter- 

 relations are very subtle : thus the problem of getting rid of Bilhar- 

 ziasis in Durban is easier than it is in Japan; for in South Africa 

 the only host of the formidable adult parasite is man, whereas in 

 Japan it also occurs in cattle. In both countries the juvenile stages 

 are spent in various kinds of water-snails, and their abundance or 

 rarity in turn is correlated with the presence of water-birds which 

 feed upon them and of water-pJants on which they feed. In some 

 cases introduced plants bring an association of animals in their 

 train. Thus Hedley notes for New Zealand that an introduced 

 "weed-florula" is able, with man's acquiescence or help, to oust the 

 indigenous flora, and that a "weed-faunula" (mouse, sparrow, 

 snail, etc.) may similarly operate against the indigenous fauna. 

 On the other hand, the introduction of hive-bees may greatly 

 improve the yield of fruit by securing pollination. 



THE CASE OF NEW ZEALAND.— The peculiar value of New 

 Zealand in reference to the problems of naturalisation is that the 

 introduction of the majority of the non-indigenous larger animals is 

 more or less definitely known. (See G. M. Thomson's masterly study, 



