330 DARWINISM AND THE PROBLEMS OF LIFE 



The transport of animals into distant lands is often 

 very useful for their extension. Especially when the 

 wanderers reach an island that offers them a virgin 

 habitat one that has few or no animal species in it 

 they may spread in all directions. And as there are all 

 kinds of spots in the new district, their descendants 

 in turn may separate into new species, each locality 

 modifying its inhabitants in a different respect. This 

 will happen particularly to slow-moving animals, such 

 as the snails, every valley and every wood giving a 

 specific modification to its inhabitants. There is no 

 reason to fear the levelling of the new characters by 

 crossing with their kind of other districts in the case 

 of such slow animals, as they have long coupled with 

 others like themselves before they reach a different 

 territory. 



There are many other means of isolation besides 

 these. The isolation may only hold during the period 

 of reproduction ; birds, for instance, that otherwise 

 mingle together in quest of food may rear their young 

 in remote districts. In such cases a differently coloured 

 ground may change the colour of the animals. It was 

 an isolation of this kind that gave rise to the migratory 

 birds. The isolation need not always involve an 

 absolute separation of the daughter species from the 

 parental species. It is enough if one part of a species 

 leaves its proper territory, and takes up residence in a 

 contiguous one of different characters, that meets its 

 needs. Although there is still a good deal of crossing 

 at the frontier, there will as a rule be a steady and pure 

 development of the new species away from it. 



