260 J". L. Bonlwte: 



method by which on the approach of winter the Stoat 

 assumes his white dress, is (although the change is from 

 brown to white) again conducted along precisely similar lines. 



Now these " poecilomeres " being differentiated in colour 

 from those parts of the body immediately surrounding them 

 it follows, and will be easily apparent, that for the same 

 reason they will often form the distinctive characters and 

 markings of many species, so that throughout the Avian 

 kingdom these tracts may be traced from being the 

 distinguishing characteristics of well-marked species to 

 inconspicuous, though permanent, markings, to markings 

 visible only during a change of plumage, or, finally, only 

 occurring as sporadic variations. 



"We noticed above (p. 256) that hybridisation tended to let 

 loose a flood of variation, and in view of the fact that these 

 " poecilomeres " occur as variations in a state of nature, it 

 was not at all surprising*, though none the less significant, 

 that all the variations appearing among the hybrids were 

 found to start on one or more of these "poecilomereSc." 



To take a few examples : in the Spotbill-Mallard trigen 

 we find the first trace of the white, which in trigen s frequently 

 spreads all over the underparts, on the fore-end of the 

 sternum, or again in the eclipse plumage of the Light Pin- 

 tail trigen (P.M.S.) we find the only spotted part to be the 

 vent " poecilomere." On the head of the second generation 

 Light Pintail trigen the crown of the head has less sheen on 

 it than the part between the eye and the ear — a point which 

 we find still more clearly and distinctly marked in the next 

 generation. Now the metallic part joining the e} T e and ear 

 represents the tract formed by the junction of the post- 

 orbital and auricular " poecilomeres " (which we have already 

 noted as occurring in a transitional state in the young male 

 Shoveler, and permanently in the adult male Teal), while the 

 browner portion along the top of the head represents the 

 tract formed by the junction of the nasal and crown 

 poecilomeres, and is found permanently in the drake Pintail 

 (Dafila acuta). So that in these cases, although we have 

 resemblances to other species, the resemblances might 

 ecmally well be considered as variations following the lines 

 of the "poecilomeres." 



