GENERAL HABITS 209 



Crossbill (Loxia curvirostra) and the Parrot 

 Crossbill (Loxia pityopsittacus) is believed to be 

 entirely due to the latter bird feeding on much 

 harder food in the Scandinavian fir forests, 

 whilst the former attacks smaller seeds and fruits. 

 The researches of Messrs. Allen and Wallace 

 have very clearly proved that the amount of 

 variation usually reaches ten or twenty, or even 

 twenty-five per cent, of the average size of the 

 varying part, whilst from five to ten per cent, 

 of the specimens examined present nearly as 

 large an amount of variation. It is by means 

 of this ever-present variation in every part of 

 a bird (internal as well as external) that natural 

 selection is able to work, and thus to harmonise 

 it with its constantly changing conditions of 

 existence in the struggle for life. 



Among birds there are two very distinct kinds 

 of Dimorphism. By this latter term is meant 

 that in certain species constant and well-defined 

 differences, either in form or colour, are found 

 between individuals of the same species. First, 

 then, we have that dimorphism which is entirely 

 dependent upon secondary sexual characters ; 

 and, second, the dimorphism which may occur 

 in either sex without any distinction. Cases of 

 sexual dimorphism are so frequent amongst 



