A REVISION OF THE GENDS GENNjEUS. 659 



be conuected with certain well-defined geographical and climatic 

 factors. 



The first type is that of the Kalij Pheasants, in which black on 

 the Tipper plumage is the prevailing tint in the males ; secondly, we 

 have the birds of the linecdus, or Burmese Silver Pheasant, group, 

 in which the upper plumage is grey, this tint being obtained by fine 

 lines and vermiculations of black and white ; and, thirdly, we have 

 the nycthemerus or Chinese Silver Pheasant group, in which white 

 greatly preponderates over the black. 



There are thus three well-defined and constant forms to be found 

 in three equally well-defined areas, but between these three forms 

 and in intervening areas we come across numerous others, more or 

 less stable, which link them together. 



An examination of the country inhabited by the three principal 

 forms shows that the following geographical and climatic factors 

 appear to be the ones which are mainly instrumental in the deter- 

 mination of the characteristics peculiar to each species. 



Thus we find that the black and very dark birds inhabit areas of 

 dense forest at comparatively low elevations where there is a heavy 

 rainfall, and which are therefore well provided with rivers, lakes and 

 swamps. Next we see that the grey birds inhabit hills of moderate 

 height covered with mixed forest, bamboos and grass lands, and 

 with a moderate rainfall. Thirdly, we obtain the white birds only 

 in hills and plateaus at a considerable elevation where the grass- 

 covered and open country exceeds in extent the forest, and where 

 the rainfall is light or even scanty. 



We thus have it demonstrated that great humidity and heat, with 

 its constant ti'opical growth of vegetation, induces black in the 

 plumage of the birds of this genus, whereas the coldness of the 

 higher mountains combined with a drier atmosphere and its conse- 

 quent thinner forests and more open grass lands induces white. 

 These four factors, temperature, humidity, elevation, and vegetable 

 growth, we shall find, therefore, are the principal ones governing 

 not only the differentiation of the species, but also of the inter- 

 mediate sub-species. An examination of the maps accompanying 

 this article will show how this reasoning is borne out, and will 

 make it easier to understand. 



It would seem very probable that in this genus the oldest form is 

 Jiorsfieldi, a species which has established itself over a very wide 

 area, extending from the West of Assam right away to the North- 

 West of the Shan States, wherever the valleys of the rivers afford it 

 sufficient heat, humidity and cover. To the West we find it re- 

 placed by certain other forms, albocristatus, leucomelanus and onela- 

 oionotus, birds which are very similar in general appearance, but 

 with more Avhite in the plumage of those which ascend to higher 

 elevations. The differences in rainfall and vegetation are not, 



