314 SIGNIFICANCE OF SEXUAL REPRODUCTION [V. 



certain species of animal and another which serves as its prey, 

 where each strives to be the quicker, so that the speed of both 

 is increased to the greatest possible extent in the course of 

 generations. Nor do they stand in the same relation as that 

 obtaining between an insectivorous bird and a certain species 

 of butterfly which forms its principal food ; in such a case two 

 totally different characters may be continually increased up to 

 their highest point, e.g. in the butterfly similarity to the dead 

 and fallen leaves among which it seeks protection when pur- 

 sued, in the bird keenness of sight. As long as the latter 

 quality is still capable of increase, so long will it still be advan- 

 tageous to any individual butterfly to resemble the leaf a little 

 more completely than other individuals of the same species ; 

 for it will thus be capable of escaping those birds which possess 

 a rather keener sight than others. On the other hand, a bird 

 with rather keener sight will have the greatest chance of catch- 

 ing the better protected butterflies. It is only in this way that 

 we can explain the constant production of such extraordinary^' 

 similarities between insects and leaves or other parts of plants. 

 At every stage of growth both the insect and its pursuer are 

 completely adapted to each other ; i. e. they are so far protected 

 and so far successful respectively, as is necessary to prevent 

 that gradual decrease in the average number of individuals 

 which would lead to the extermination of the species ^ But 

 the fact that there is complete adaptation at each stage does 

 not prevent the two species from increasing those qualities of 

 protection and of pursuit upon which they respectively depend. 

 So far from this being the case, they would be necessarily 

 compelled to gradually increase these qualities so long as the 

 physical possibility of improvement remained on both sides. 

 As long as some birds possessed a rather keener sight than 

 those which previously existed, so long would those butterflies 

 possess an advantage in which the resemblance to leaf-veining 

 was more distinct than in others. But from the moment at 

 which the maximum keenness of eyesight attainable had been 

 reached, at which therefore all butterflies resembled leaves so 

 completely that even the birds with the keenest eyesight might 



^ In order to make the case as simple as possible, I assume that the 

 insectivorous bird feeds upon a single species of insect, and that the 

 insect is only attacked by a single species of bird. 



