252 TJic World-Evidence. 



he finds a young cow-bird in a nest, he kills or drowns 

 it ; or, when he finds an egg, he destroys it. 



One point on which we would fain have more defi- 

 nite and reliable information about the cow-birds — 

 results of exact observation and comparison — is, as 

 to the disparity of the sexes in numbers. There is a 

 great difference between three to one, and ten or nine 

 to one. This is, to our mind, an essential element in 

 the study of parasitic birds. We believe, as surely 

 as we now write, that the source and origin of para- 

 sitism is to be found here in polyandrous promiscuity ; 

 and the degree to which that has advanced, owing to 

 permanent disproportion of the sexes, is the measure 

 in which parasitism among birds has proceeded. The 

 two are related to each other, as cause and effect ; 

 though, indeed, conditions of culture, and changes 

 effected by man, may be a second or collateral cause, 

 operating to aid the other. 



