Natural Selection 147 



material" whereon to work. Hence the origin of 

 species has been ascribed by some to an innate direct 

 tendency in animals to vary in certain definite direc- 

 tions. It has been pointed out how, among butter- 

 flies, regular modification in the wing-patterns can 

 be traced through a succession of nearly-related 

 species. And as these differences in pattern seem 

 to be of no use to the insects in the struggle for life, 

 they are supposed to be due to an inherent tendency 

 to vary along definite lines, under stimulation from 

 outside influences — such as changes in climate (po). 

 The nature of variation still calls for a vast amount 

 of study, but it seems impossible to claim either that 

 variation is always indefinite or that it is always 

 necessarily determined along certain fixed lines. 



In the preceding pages an attempt has been made 

 briefly to put forward the various current explana- 

 tions of the origin of species. While the author of 

 this little book believes that the Darwinian theory is 

 largely supported by facts, and that the alternatives 

 which have been proposed to supersede it rest to 

 a great extent on unproved theories, he cannot sub- 

 scribe to the " all-sufficiency " of natural selection. 

 The insect-world presents us with such varied and 

 complicated features that it is hard to believe that 

 the origin of its myriad kinds can be explained by 

 any one agency. 



Prevention of Intercrossing. — One other point 

 demands attention before we pass from the question 

 of the origin of species. An objection which has 

 been urged against the development of species out 

 of varieties is that the incipient species would be 

 "swamped "by intercrossing with the parent form. 

 Varieties of the same species usually breed freely 

 among themselves, but hybrids between two distinct 

 species are rare in nature, and when they do occur 



