The Making of Species 
trangebarica). 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 Chloéphaga, in 
which some species, as the familiar Upland or 
Magellan Goose of our parks (C. magellanzca), 
have the sexes utterly unlike, while in others, as 
the Ruddy-headed Goose (C. rudzdiceps), 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 (Aas doscas) 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 (Azas pecelorhyncha), the Australian 
Grey Duck (4. supercztiosa), 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 ({/7pfotragus niger and 
H.. equinus) furnish familiar examples of nearly- 
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