306 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



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

 keener sight will have the greatest chance of catching 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 extermina- 

 tion of the species l . 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 respec- 

 tively 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 dis- 

 tinct than in others. But from the moment at which the maximum 

 keenness of eyesight attainable had been reached, at which there- 

 fore all butterflies resembled leaves so completely that even the 

 birds with the keenest eyesight might fail to detect them when at 

 rest, from this very point any further improvement in the simi- 

 larity to leaves would cease, because the advantage to be gained from 

 any such improvement would cease at the same time. 



Such reciprocal intensification of adaptive characters appears to 

 me to have been one of the most important factors in the transfor- 

 mation of species : it must have persisted through long series of 

 species during phylogeny: it must have affected the most diverse 

 parts and characters in the most diverse groups of organisms. 



In certain large butterflies of the Indian and African forests 

 Kallima paralecta, K. inac/ns, and A", albqfasciata it has been 

 frequently pointed out that the deceptive resemblance to a leaf is so 

 striking that an observer who has received no hint upon the subject 

 believes that he sees a leaf, even when he is looking at the butter- 

 fly very closely. The similarity is nevertheless incomplete ; for out 



1 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. 



