484 DE. ALT-RED E. WALLACE Olf 



affects the species. It irmst, therefore, undergo some amount of 

 modification, either structural or functional, in order to succeed 

 under the new conditions ; and the constant variations of every 

 part around its mean furnish the materials for adapting the 

 organism to these new conditions. If a new enemy is the 

 danger to be guarded against, this adaptation may be effected in 

 several ways. Swiftness in running or flying, habits of conceal- 

 ment, 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 

 inportant characteristics. These are (1) some amount of infer- 

 tility 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 considerable distances or when partially concealed ; 

 or, in the case of flowering plants, be distinguished by the insects 

 which fertilize 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 continental 

 areas. But this danger is evidently much increased if crossing 

 with the parent form is not at first checked and soon afterwards 

 completely prevented, 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 importance 

 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 



