Variation and Natural Selection. 101 



short, smooth tail. The segregation of these two species 

 has probably been effected by the difference of their mode 

 of life, and each has been adapted to its special environment 

 through the elimination of those individuals which were 

 not in harmony with the condition of their life. It is 

 probable that this mode of segregation has been an im- 

 portant one. And it is clear that in many cases competition 

 would be a co-operating factor in this process, weaker 

 organisms being forced into otherwise uncongenial habitats 

 through the stress of competitive elimination, the weaker 

 forms not perishing, but being eliminated from more 

 favoured areas. 



Protective coloration may also be a means of segrega- 

 tion. A species of insects having no protective resemblance 

 might vary in two directions — in the direction of green 

 tints, assimilating their hue to that of vegetation ; and in 

 the direction of sandy or dull earthy colours, assimilating 

 them to the colour of the soil. In the one variety elimina- 

 tion would weed out all but the green forms, and these 

 would be left to intercross. In the other variety, green 

 forms would be eliminated, dull-brown forms being left to 

 interbreed. Stragglers from one group into the other 

 would stand a chance of elimination before interbreeding 

 was effected.* 



In the case of birds whose freedom of flight gives them 

 a wide range, sometimes almost a world-wide range, it 

 would seem at first sight that their facilities for inter- 

 breeding and intercrossing are so great that divergence is 

 well-nigh impossible. And yet the examples of divergence I 

 cited from Mr. Wallace were taken from birds, and it is well 

 known that divergence is particularly well shown in this 

 class. But when the habits of birds are studied attentively, 

 it is found that, wide as is their range, their breeding area 

 is often markedly restricted. The sanderling and knot 



* From the absence of interblending in some cases (to be considered 

 shortly), both brown and green forms may be produced ; and under certain 

 circumstances, even a power of becoming either brown or green in the 

 presence of appropriate stimuli. 



