262 J. L. Bonhote: 



a definite and wide-spreading law we have, in my opinion, a 

 far more reasonable and adequate explanation of the facts. 



In another sense, again, " reversion in the abstract " may 

 occur, and by this we mean the general tendency shown 

 among these hybrids for the plumage of the sexes to become 

 more and more alike, and what follows therefrom, viz., the 

 suppression of the brighter plumage of the Drake and 

 elimination of the eclipse plumage, but this is a matter about 

 which oui- data are as yet insufficient. 



The only remaining point to be noticed is that of the 

 " white coloration " which has appeared in so many cases. 

 It is first apparent as a sternal poeciloinere in the Mallard- 

 Spotbill digens and in the Mallard-Spotbill-Pintail trigens of 

 first generation we find it very much more marked and 

 covering the whole of the underparts. In the second genera- 

 tion of the same cross it is visible on the sides of the face as 

 well, although in this case the white is not pure, but buffish, 

 with dark mottlings, and the third generation we find to be still 

 lighter. In some cases the orbital " poecilomeres " in the 

 females are clearly marked in white. 



Now what is the explanation of this white colour ? At 

 first sight from the complete white underparts of the Pintail 

 trigens one would be tempted to say that it was merely due 

 to the Pintail, but as it occurs in other crosses which 

 have no Pintail in their parentage, and as moreover when 

 it appears it is common to all ages and both sexes, some other 

 explanation must be sought. Now we notice that the more 

 inbred these Pintail trigens become, the lighter they are, and 

 further I have just reared an extremely light brood of 

 Mallard-Spotbills which are also much inbred, and this renders 

 it extremely probable that the light colour is due to lack of 

 vigour caused by inbreeding. In my paper to the Linnsean 

 Society, quoted above, I gave many instances of lack of 

 vigour producing white coloration among birds in captivity, 

 and I believe that in the case of these hybrids lack of vigour 

 is the cause of the white, which one may again notice as 

 always occurring first of all on the poecilomeres. 



The matter, however, has a further complication ; if the 

 white is, as I allege, due to lack of vigour, why is it that the 

 white Pintail trigens are fertile inter se, while so far I have 



