274 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



method, as in many species of Cynips and in certain lower Crustacea. 

 But all these differ from our hypothetical case in one important 

 respect ; it is always impossible for all the descendants to reach 

 maturity and reproduce themselves. The vast majority of the 

 descendants generally perish at an early stage, and only about as 

 many remain to continue the species as reached maturity in the 

 preceding- generation. 



We have now to consider whether such a species can be subject 

 to the operation of natural selection. Let us take the case of an 

 insect living among green leaves, and possessing a green colour as 

 a protection against discovery by its enemies. We will assume that 

 the hereditary individual differences consist of various shades of 

 green. Let us further suppose that the sudden extinction of its 

 food-plant compelled this species to seek another plant with a 

 somewhat different shade of green. It is clear that such an insect 

 would not be completely adapted to the new environment. It 

 would therefore be compelled, metaphorically speaking, to en- 

 deavour to bring its colour into closer harmony with that of the 

 new food-plant, or else the increased chances of detection given to 

 its enemies would lead to its slow but certain extinction. 



It is obvious that such a species would be altogether unable to 

 produce the required adaptation, for ex hypotkesi, its hereditary 

 variations remain the same, one generation after another. If 

 therefore the required shade of green was not previously present, 

 as one of the original individual differences, it could not be pro- 

 duced at any time. If, however, we suppose that such a colour 

 existed previously in certain individuals, it follows that those with 

 other shades of green would be gradually exterminated, while 

 the former would aloUe survive. But this process would not be 

 an adaptation in the sense used in the theory of natural selection. 

 It would indeed be a process of selection, but it could form no 

 more than the beginning of that process which we call natural 

 selection. If the latter could only bring existing characters into 

 prominence, it would not be worth much consideration, for it could 

 never produce a new species. A species never includes, from the 

 beginning, individuals which deviate from the specific type as 

 widely as the individuals of the most nearly allied species 

 deviate from it. And it would be still less possible to explain, 

 on such a principle, the origin of the whole organic world ; for, if 



