THE PSOBLEM OF UTILITY. 485 



injurious. I have endeavoured to show, and can still find no flaw 

 in my reasoning, that 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 tbe rapidity of 

 adaptation *. This view will explain the curious fact of the well- 

 marked differences of colour or form which, almost invariably 

 characterize allied species. These '• recognition marks," as I bave 

 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 differentiation, 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 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 extiuct 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) sur- 

 vival of the fittest would begin to act, and the differences of 

 food and climate, with the different kinds of enemies, would render 

 some modifications 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 continent. In 

 both these cases recognition-characters, to aid in the prevention 

 of intercrossing, would be produced 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, 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 would need to be differentiated. 

 Neither is the small amount of divergence that usually prevails 

 between the mean of a few individuals taken at random, such as 

 might have accidentally reached an island, and the average type 



* ' Darwinism,' pp. 174-180. 



