WILD HYBRIDS 197 



making the usual rounded spot, but it merges by a gradual black cloudiness into the 

 distal metallic gloss. In fact, the tail colouring in this individual closely resembles 

 mantchuricum. 



The extreme variability of the rectrice white would seem to indicate the worthlessness 

 of this character as a means of specific differentiation. 



Crossoptilon drouynii 



The type of this so-called species is in the Mus^e d'Histoire Naturelle, in Paris. 

 The entire body plumage is white. The secondaries are slightly shaded with grey, and 

 the primaries clouded on the inner webs with grey. The shafts of all the flight feathers 

 are dark brown. The tail-feathers are twenty in number, greyish-white at the base, 

 shading on the terminal half into metallic bronze-green and deep purple. 



In tibetanum with a slight admixture of auritum blood we thus find a pronounced 

 tendency to variation in the white of the tail-feathers. The first hints of this occur in 

 the form of small elongated spots on the outer webs of the lateral rectrices. About 

 twenty per cent, of the males show this phase, and in the type of tibetanum these spots 

 are well developed, though asymmetrically, on the six outer pairs. Crossoptilon drouynii 

 was based on an individual with greyish-white rectrices. 



C. leucurum shows extreme variation, no two individuals in the museums of 

 England, France and elsewhere being alike, and many showing very great differences. 

 This is true, as we have seen, not only as regards colour and pattern, but even in the 

 number of rectrices, there being twenty in some individuals, and twenty-two in others. 



The only logical solution of this tangle is to consider harmani, leucurum and 

 drouynii as hybrids and sink them in the synonomy, the first of auritum and the two 

 latter of tibetanum, according to whether the blue or white colouring predominates. 



The variation in number of tail-feathers in Crossoptilon is of interest. In tibetanum 

 there are twenty, and all with vanes quite solid and normal in structure. The whiter 

 leucurum-likt birds show no change, but in most of those individuals which approach 

 harmani and auritum, an extra central pair of highly disintegrated feathers appears, 

 above the others, suggesting, from their position, derivation from the upper tail-coverts. 

 In auritum still another pair is present, making twenty-four in all, this additional pair 

 also being central, superior, and much disintegrated. So the specialization is definite as 

 to position. In more than one bird, which in colour from beak to tail is typical auritum, 

 I have found after careful macro- and microscopical examination only twenty-two 

 rectrices ; none having been lost accidentally, but one of the central pairs being 

 congenitally absent. These birds were unquestionably hybrids with the lessened number 

 of rectrices as the sole indication of their mixed blood. 



In the rather isolated, more generalized, brown mantchuricum, twenty-two is the 

 normal number, and here we find but a single pair of central, superior disintegrated tail 

 feathers, showing that the locus of specialization is the same as in the other species. 



In the presence or absence of certain rectrices in these birds we encounter another 

 of those unexpected correlations which meets the student of avian evolution at every step. 

 As regards colour there is no doubt but that the snow-white tibetanum birds are by far the 



