GENERAL HABITS 207 



motionless until the threatened danger has 

 passed. In these situations they often so closely 

 resemble a stone, a clod of earth, an excrescence 

 on the bark, a heap of leaves, or the stalk and 

 leaves of surrounding plants, that discovery is 

 next to impossible. 



We will bring the present chapter to a close 

 with a few remarks on the somewhat compli- 

 cated and at present but little understood sub- 

 jects of Variation and Dimorphism. Possibly 

 few readers even remotely suspect to what an 

 extent variation occurs among birds. The fact 

 can only be realised by the actual examination 

 of large series of individuals of the same species. 

 Variation, as most persons know, is a very com- 

 mon phenomenon amongst domesticated birds 

 and animals ; it may never be so pronounced 

 in wild birds, because of the strong tendency 

 of natural selection to prevent it, but it is 

 present in all to a degree that is not positively 

 harmful to the species. When, however, it may 

 reach an exceptionally pronounced type, such 

 as in the case of an albino bird, it is invariably 

 stamped out owing to its want of harmony with 

 its environment. It is therefore no exaggera- 

 tion to say that no two birds are exactly alike ; 

 the more minute the examination, the greater 



