144 HEREDITY AND SELECTION IN SOCIOLOGY 



It is still more propitious to the evolution of species if aa 

 isolated territory be large enough to afford sustenance, not only 

 for the originally transported species, but for others nearly 

 related. In this case, the original species can adapt itself to a 

 number of different environmental conditions, and a number of 

 new species, not merely of new varieties, can be formed. This 

 has been the case with those land-snails of the island of Celebes 

 which we have already mentioned ; as the original emigrant 

 multiplied its descendants, these distributed themselves more 

 and more over the island, and were compelled to adapt them- 

 selves to an ever-increasing number of new conditions ; with 

 the result that a number of new species were formed, differing 

 among themselves, however, only in comparatively trivial 

 features, such as we are accustomed to find among the varieties 

 of the same species. We have, in the case of the Celebes snails, 

 a good example of the co-operation of isolation and natural 

 selection, the latter selecting those individuals most adaptable 

 to the new environment. Darwin has expressed the view that 

 " if an isolated area be very small, either from being surroimded 

 by barriers, or from having very peculiar physical conditions, 

 the total number of the inhabitants will be small ; and this wiU 

 retard the production of new species through natural selection, 

 by decreasing the chances of favourable variation arising." 

 And, further, that " although small isolated areas have been in 

 some respects highly favourable for the production of new 

 species, yet the course of modification wiU generally have been 

 more rapid on large areas ; and what is more important, the 

 new forms produced on large areas, which have already been 

 victorious over many competitors, will be those that will spread 

 most widely, and will give rise to the greatest number of new 

 varieties and species." ^ 



Thus, isolation must be regarded as an important factor in 

 the transformation of species. Its action will be all the more 

 1 The Origin of Species, pp. 128 ff. 



