cHAP, 111.] CONDITIONS AFFECTING DISTRIBUTION. 45 
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 
Crigin 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 
