v.] IN THE THEORY OF NATURAL SELECTION. 28 1 



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 com- 

 pletely adapted to the new environment. It would therefore 

 be compelled, metaphorically speaking, to endeavour 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 hypothesi, its heredi- 

 tary 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 produced 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 alone 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 prin- 

 ciple, the origin of the whole organic world ; for, if so, all 

 existing species would have been included as variations of the 

 first species. Natural selection must be able to do infinitely 

 more than this, if it is to be of any importance as a principle of 

 development. It must be able to accumulate minute existing 



