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 



O 



