CHAPTER V. 



GEOGRAPHICAL DISTRIBUTION. 



THE geographical distribution of animals on the 

 world to-day is another source of collateral evidence 

 as to the origin of species. It is of particular interest, 

 since it was the study of this subject which first led 

 Darwin to the theories which have made his name 

 immortal. It was this same class of facts also, accu- 

 mulated by years of travel, which led Alfred Wallace 

 independently to conclusions similar to those of 

 Darwin. To-day, it is still the subject of geograph- 

 ical distribution which offers some of the most con- 

 vincing proofs of the modification of species, while 

 at the same time it presents some of the most puz- 

 zling problems for solution. 



The subject is one of the greatest complexity, for 

 not only are the species to be studied very numerous, 

 but the various causes acting in the past and present 

 to alter the distribution of any species are almost 

 infinite. Consider how little it takes to produce a 

 change in the fauna and flora of a given locality. 

 The introduction of a few English sparrows into the 

 United States has, in a very few years, caused great 

 changes in our animals, driving away and almost 

 exterminating some American birds. The increase 

 in the cultivation of the potato has, in a short time, 

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