INFLUENCE OF ISOLATION ON THE FORMATION OF SPECIES 283 



land-birds, among them a duck, a Ijuzzard, and about a dozen different 

 but nearly related mocking-birds, each of wliich is found in only one 

 or two of the fifteen islands. The group of islands also possesses 

 peculiar reptiles, and they take their name from the gigantic land- 

 tortoises, sometimes 400 kilogrammes in weight, wliich in Tertiary 

 times inhabited also the continent of South America, but are now 

 found in the Galapagos Islands only. The islands also possess 

 endemic lizards of the genus Tropidurus, and although the lizards 

 can no more have been transported across the ocean than the tortoises, 

 but corroborate the conclusion drawn from geological data, that the 

 islands were still connected with the mainland in Tertiary times, 

 the occurrence of a particular species of Tropidurus upon almost 

 every one of the fifteen islands testifies anew to the mysterious 

 influence of isolation, for most of these islands are quite isolated 

 regions for the difierent species of lizard, even more than for the 

 mocking-birds, which have also split up into a series of species. 



We are thus led to the hypothesis, whicli was first introduced 

 into the Evolution Theory by Darwin, that the prevention of constant 

 crossing of an isolated colony with the others of the same species 

 from the original habitat favours the origin of new endemic species, 

 and his conclusion is confirmed when we learn that islands like the 

 Galapagos group possess twenty-one endemic. land-birds, but only two 

 endemic sea-birds out of eleven, for the latter traverse great stretches 

 of sea, and crossings with others of the same species on the neighbour- 

 ing continental coasts will often take place. The Bermuda Islands 

 also afford a proof that the development of endemic species is 

 prevented by regular crossing with other members of the si:>ecies 

 from the original habitat, for although they are 1,200 kilometres 

 distant from the continent of North America — that is, further than the 

 Galapagos Islands from South America — tliey possess no endemic 

 species of bird, and we may undoubtedly associate this with the fact 

 that the migratory birds from the continent visit the Bermudas every 

 year. 



Madeira also confirms our conclusion, for only one of the 

 ninety-nine species of bird occurring there can be regarded as endemic, 

 and it has often been observed that lairds from the neighbouring 

 African mainland (only 240 kilometres distant) are driven across 

 to Madeira. Terrestrial snails, on the other hand, will seldom be 

 carried to Madeira by birds, and accordingly we find there an 

 extraordinary number of endemic terrestrial snails, namel}^ 109 species. 



Although these and similar facts indicate strongly that isolation 

 favours the evolution of new species, it would be erroneous to imagine 



