The geographical distribution of animals, with a study of the relations ...

chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 46come in, while many others would be unable to live there ; andthe immediate cause of this great alteration would probably bequite imperceptible to us, even if we could watch it in progressyear by year. So, in South Africa, the celebrated Tsetse flyinhabits certain districts having well defined limits ; and whereit abounds no horses, dogs, or cattle can live. Yet asses,zebras, and antelopes are unaffected by it. So long as this flycontinues tc exist, there is a living barrier to the entrance ofcertain animals, quite as effectual as a lofty mountain rangeor a wide arm of the sea. The complex relations of one formof life with others is nowhere better illustrated than in Mr.Darwin's celebrated case of the cats and clover, as given in hisOrigin of Species, 6th ed., p. 57. He has observed that bothwild heartsease and red-clover are fertilized in this country byhumble-bees only, so that the production of seed depends onthe visits of these insects. A gentleman who has speciallystudied humble-bees finds that they are largely kept down byfield-mice, which destroy their combs and nests. Field-micein their turn are kept down by cats ;and probably also by owls;so that these carnivorous animals are really the agents in renderingpossible the continued existence of red-clover and wildheartsease. For if they were absent, the field-mice having noenemies, would multiply to such an extent as to destroy, all thehumble-bees ; and these two plants would then produce noseed and soon become extinct.Mr. Darwin has also shown that one species often exterminatesanother closely allied to it, when the two are brought intocontact. One species of swallow and thrush are known tohave increased at the expense of allied species. Rats, carriedall over the world by commerce, are continually extirpatingother species of rats. The imported hive-bee is, in Australia,rapidly exterminating a native stingless bee. Any slight change,therefore, of physical geography or of climate, which allowsallied species hitherto inhabiting distinct areas to come intocontact, will often lead to the extermination of one of them; andthis extermination will be effected by no external force, by noactual enemy, but merely because the one is slightly better