INFLUENCE OF ISOLATION 145 



beneficial when the isolated area is large enough to support a 

 number of species, and when the original species can difEerentiate 

 itself into several new species, according to the difierences in 

 climatic and other conditions obtaining in the area. An isolated 

 area, if it be too small, inhibits further difEerentiation ; but a 

 larger area, capable of affording sustenance to a number of species, 

 wiU be doubly advantageous : firstly, because isolation from the 

 parent species permits of greater development of variations ; and, 

 secondly, because the mutual influence exerted by the isolated 

 species, and the greater necessity for rigorous adaptation, will 

 involve a keener competition, and, consequently, a stronger set 

 of survivors.^ 



^ Darwin has shown that many animals and plants which we might at 

 first think could not be transported from one country to another over the 

 sea are, nevertheless, so transported. We can understand birds and 

 butterflies and bats being blown by the wind across vast tracts of sea, but 

 we could not have predicted that seeds could be transported in the in- 

 testine of a bird, or that minute animals could be carried across the ocean 

 in a small mass of earth or mud adhering to the bird's foot ; such is the case, 

 however. There is no doubt but that such passive migrations have played 

 an important part in the origui of species. 



Not only an island, but also a lofty mountaia or plateau, may form an 

 isolated area. Isolation in an absolute sense cannot be imagined, for 

 there is probably no spot entirely inaccessible under any conditions to 

 any species. And if we find species existing in any area, it is always 

 possible that others may likewise arrive. Nevertheless, isolation may be 

 complete during the whole period which our observation is capable of 

 embracing. We must also take into account the cKmatic and geological 

 conditions and the possible changes which may occur, such as the sinking 

 of the land and its washing away by the sea, and vice versa, the sudden 

 elevation of new land as a result of volcanic disturbances, etc. It is 

 possible that several endemic species of snails which are found on the 

 Sandwich Islands and Galapagos Islands were there previously to the 

 sinking of the land which formerly joined these islands with the mainland. 



The Sandwich Islands, which are some 3,000 miles from the American 

 coast, have a very interesting snail faima, consisting of not less than 

 400 endemic species ; and Weismann is of opinion that these endemic 

 species owe their origin to accidental importation by birds from th^ 

 continent. These islands possess also eighteen endemic species of birds. 

 The Galapagos Islands, which lie some 750 miles from the mainland, 

 possess twenty-one endemic species of birds. The island of Madeira 

 possesses 109 endemic species of snails, but only one single endemic species 



10 



