chap, in.] CONDITIONS AFFECTING DISTRIBUTION. 46 



come in, while many others would be unable to live there ; and 

 the immediate cause of this great alteration would probably be 

 quite imperceptible to us, even if we could watch it in progress 

 year by year. So, in South Africa, the celebrated Tsetse fly 

 inhabits certain districts having well defined limits ; and where 

 it abounds no horses, dogs, or cattle can live. Yet asses, 

 zebras, and antelopes are unaffected by it. So long as this fly 

 continues to exist, there is a living barrier to the entrance of 

 certain animals, quite as effectual as a lofty mountain range 

 or a wide arm of the sea. The complex relations of one form 

 of life with others is nowhere better illustrated than in Mr. 

 Darwin's celebrated case of the cats and clover, as given in his 

 Origin of Species, 6th ed., p. 57. He has observed that both 

 wild heartsease and red-clover are fertilized in this country by 

 humble-bees only, so that the production of seed depends on 

 the visits of these insects. A gentleman who has specially 

 studied humble-bees finds that they are largely kept down by 

 field-mice, which destroy their combs and nests. Field-mice 

 in their turn are kept down by cats ; and probably also by owls ; 

 so that these carnivorous animals are really the agents in ren- 

 dering possible the continued existence of red-clover and wild 

 heartsease. For if they were absent, the field-mice having no 

 enemies, would multiply to such an extent as to destroy all the 

 humble-bees ; and these two plants would then produce no 

 seed and soon become extinct. 



Mr. Darwin has also shown that one species often exterminates 

 another closely allied to it, when the two are brought into 

 contact. One species of swallow and thrush are known to 

 have increased at the expense of allied species. Rats, carried 

 all over the world by commerce, are continually extirpating 

 other 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 allows 

 allied species hitherto inhabiting distinct areas to come into 

 contact, will often lead to the extermination of one of them; and 

 this extermination will be effected by no external force, by no 

 actual enemy, but merely because the one is slightly better 



