382 STUDIES, SCIENTIFIC AND SOCIAL chap. 



this adaptation may be effected in several ways. Swiftness 

 in running or %ing, habits of concealment, or seeking new 

 kinds of food in places inacessible to the enemy, may each 

 lead to the survival of those individuals which were 

 sufficiently intelligent to adopt them, or sufficiently 

 favoured by rapid variation in the desired direction. 

 Survival of the fittest in these respects, going on year by 

 year, might lead to the formation of two or more diverging 

 races each able to maintain itself in the presence of the 

 new enemy, while the former average type of the species 

 rapidly became extinct. We should thus have two or three 

 incipient new species ; but they would not become well 

 differentiated species till they had acquired certain definite 

 and important characteristics. These are (1) some amount 

 of infertility when crossed with the parent form or with 

 each other ; and (2) some distinct and conspicuous external 

 characters by means of which the new varieties could 

 readily distinguish their own kind even when at consider- 

 able distances or when partially concealed ; or, in the case 

 of flowering plants, be distinguished by the insects which 

 fertilise them. 



The greatest danger to a species under new and adverse 

 conditions is, that it should not be able to adapt itself to 

 them with sufficient rapidity. It is for this reason that, as 

 Darwin concludes, new species arise, mainly, from those 

 which have a large population, which occupy a wide area, 

 and which present much variation — a combination rarely 

 found except in the great continents. But this danger is 

 evidently much increased if crossing with the parent form 

 is not at first checked and soon afterwards completely pre- 

 vented except as a quite exceptional occurrence. The means 

 of preventing this intercrossing are, for animals, either 

 infertility, external distinctions leading to the preferential 

 mating of similar forms, or physical isolation. The latter 

 I believe, with Darwin, to be of comparatively little im- 

 portance and to have very rarely been the chief agent in 

 modification. In the great majority of cases a new species 

 must arise amidst the population of an existing species ; and 

 while its adaptation is progressing any intercrossing with 

 the parent form will be injurious. I have endeavoured to 

 show, and can still find no flaw in my reasoning, that 



