COINCIDENCE AND CORRELATION 67 



spread method of protection from predatory 

 enemies. It is an extreme manifestation of the 

 virtue of immobility which is equally well known 

 to hunters and to their quarry. When it is 

 associated with singularities of form and colora- 

 tion, it produces a combination which is so 

 remarkable that it is generally received with a 

 proper amount of scepticism. The resemblance 

 which these animals bear to a leaf is a selective 

 one ; that which they bear incidentally to each 

 other is a convergent one, due to their inde- 

 pendent acquirement of a common fundamental 

 form. In these cases it may be supposed that 

 natural selection has taken advantage of a 

 previously existing aptitude, the essence of the 

 leaf-form being present as a generic character 

 before the actual assumption of the intimate 

 resemblance became fixed. 



Other striking convergences depending upon 

 form and coloration are those which obtain 

 between some harmless and some poisonous 

 snakes inhabiting the tropical regions of both 

 hemispheres. Darwin described the examples 

 from South America. The mutual resemblances 

 proceed largely from the character of the trans- 

 verse banding of the body which they have in 

 common ; and the colour coincidence may be 

 so close that in a rapid sorting the species 

 would be thrown together and confounded one 

 with another. 



Lastly, reference should be made to the well- 



