XVIII THE PROBLEM OF UTILITY 383 



mutual infertility would be usually brought about by 

 natural selection wherever the two forms were in contact, 

 and also that the early occurrence of well-marked external 

 differences would assist greatly in the rapidity of adapta- 

 tion.^ This view will explain the curious fact of the 

 well-marked differences of colour or form which almost 

 invariably characterise allied species. These " recognition 

 marks," as I have termed them, are of great use even to 

 existing well-defined species, but they must have been of 

 still greater use during the earlier stages of differentia- 

 tion, when the very existence of the new form must have 

 largely depended on them. 



I may here remark that it is because these external 

 differences of colour or marking are quite as constantly 

 present in peculiar insular species as in those inhabiting 

 a continent, that I do not believe in local isolation as of 

 any real importance in species-formation. Insular species 

 may have been produced in two ways. Either a portion 

 of a declining species may have reached the island, where 

 it survived through the more favourable conditions while 

 it became extinct on the continent ; or, a few individuals 

 of a dominant species reached the island, where, owing to 

 the absence of competition, they rapidly increased till the 

 island became fully stocked with the unchanged species. 

 Then (and then only) survival of the fittest would begin 

 to act, and the differences of food and climate, with the 

 different kinds of enemies, would render some modifica- 

 tions of structure, form, or colour advantageous, and thus 

 a new species would be formed by adaptation from the 

 old one in almost exactly the same way as on the con- 

 tinent. In both these cases recognition-characters, to aid 

 in the prevention of intercrossing, would be developed by 

 natural selection. But if insular species have usually 

 been formed by a few individuals somewhat different from 

 the type having first reached the island and thereafter 

 preserved their peculiarities — as alleged by Romanes and 

 others — there is no reason why any distinctive and stable 

 form of coloration or marking should have been developed, 

 since there would be no similar species from which it 

 ^ Darwinism, pp. 174-180. 



