SUPPLEMENTARY FACTORS OF EVOLUTION 489 



of their kind. And it is particularly significant to note, in this 

 connection, that such islands are peculiarly rich in distinct species. 

 Isolation is also exemplified by the area between tide-marks, as 

 in some of the periwinkles. Some of these creatures are gradu- 

 ally becoming adapted to breathing damp air, those which are 

 best off in this respect living near high-water mark. It is pretty 

 clear that individuals varying so as to breathe damp air better 

 than their fellows would naturally take to living further from the 

 sea, and would be thus to some extent isolated. 



Isolation may also be of a physiological nature, as emphasized 

 by Romanes in his theory of physiological selection. We know 

 that, as a rule, the crosses between allied species, i.e. hybrids, 

 are infertile, and it is largely owing to this fact that animal 

 species remain distinct. It seems, therefore, a plausible assump- 

 tion that the rise of new species has partly been rendered possible 

 by an increasing tendency for the crosses between them and their 

 parent stocks to be infertile. In other words, there has been a 

 physiological variation in the direction indicated, alongside of 

 other variations in shape, proportion, colour, &c. &c. 



SUPPLEMENTARY FACTORS OF EVOLUTION 



Admitting the importance of Natural Selection, it by no means 

 follows that it has been the only evolutionary factor determining 

 the origin of new species. 



Courtship Selection. — Darwin believed that some of the 

 characters of male animals have been brought about by selec- 

 tion exercised on the part of their mates. The possibilities in 

 this direction have already been discussed at some length (see 

 p. 143) in dealing with the Law of Batde and the Law of 

 Beauty. It is, after all, a special kind of Natural Selection, 

 which may have determined the evolution of certain weapons and 

 of aesthetic characters. 



Lamarckism. — Under this head may be included the pre- 

 Darwinian views of the French naturalists Lamarck and Buffon, 

 to which Darwin himself was inclined to attach some importance. 

 These views turn upon the inheritance of "acquired characters", 

 regarding which there has been an interminable amount of 

 discussion. It must be premised that the body of an animal 

 higher in the scale than an Animalcule is related to (i) 



