EVIDENCE FROM DISTRIBUTION 121 



seem that distribution were wholly a matter of 

 climate. But this explanation fails on closer exam- 

 ination. The tropical parts of Australia, Africa and 

 South America have very similar climates and yet 

 it would be difficult to select three regions of the 

 earth which are more radically distinct from the 

 zoological point of view. The fact that a given 

 animal or plant does not occur in a particular region, 

 is no conclusive proof that it is not fitted to live 

 there. The rabbits introduced into Australia have 

 multiplied to such an extent that they have become 

 a very serious pest, as have the European Gypsy and 

 Brown-tail Moths in Massachusetts. The acclima- 

 tization of the Indian Mongoose in Jamaica has 

 upset the whole zoological equilibrium of the island, 

 while domesticated horses and cattle, running wild, 

 grew into vast herds, which overspread the plains of 

 North and South America. European weeds are 

 among the most difficult and annoying enemies 

 with which the American farmer has to contend, 

 and the American cactus, incautiously brought into 

 South Africa, has become a nuisance and taken pos- 

 session of large areas. Cactuses and the Agave (or 

 Century Plant, also American) have taken so kindly 

 to the Mediterranean lands, that many artists have 

 painted them in scenes of classical antiquity, which 

 to the botanical geographer is amusing. These 

 and many similar facts sufficiently show that the 

 introduction of a new animal or plant may be a very 

 serious undertaking, fraught with great possibilities 



