The Making of Species 



tranqebarica). The habits of all these four 

 species appear to be identical, nevertheless in 

 the first three the sexes show little or no dis- 

 similarity in outward appearance, while in the 

 last the sexual dimorphism is so great that the 

 cock and hen were formerly thought to belong to 

 different species. 



Another very curious case is that of the South 

 American geese of the genus Chloephaga, in 

 which some species, as the familiar Upland or 

 Magellan Goose of our parks (C. magellanica), 

 have the sexes utterly unlike, while in others, as 

 the Ruddy-headed Goose (C. rubidiceps\ they are 

 quite similar to each other. 



The ducks furnish us with another very good 

 example of the apparently haphazard nature of 

 sexual dimorphism. In the Common Mallard or 

 Wild Duck (Anas boscas] the cock is far more 

 showily coloured than the hen, but in all the 

 species most nearly allied to it the males are as 

 inconspicuous as the females, e.g. in the Indian 

 Spotted-bill (Anas pozcilorhynchd), the Australian 

 Grey Duck (A. superciliosa), the African Yellow 

 Bill (Anas undulata), and the American Dusky 

 Duck (A. obscura). As the dusky duck inhabits 

 North America, where the mallard is also found, 

 the case is particularly striking. 



Among mammals the lion and the tiger and the 

 sable and roan antelopes (Hippotragus niger and 

 H. equinus) furnish familiar examples of nearly- 



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