SElTEMliLR 15, 1910] 



NATURE 



329 



An Undescribed Feather Element. 



Iv all the European ducks, geese, and swans, and in 

 certain game birds, tliere is a remarkable feature about 

 the structure of the primary feathers that seems to be 

 hitherto undescribed. The under surface of a feather 

 from such a bird bears a distinct glistening " mirror " 

 occupying that portion of the web adjoining the rhachis. 

 It is quite visible to the unaided eye in any position of 

 light, and may be readily detected by the finger-tip. A 

 closer e.vamination shows this area to consist of a series 

 of narrow silvery (sometimes golden or brassy) membranes 

 each closely overlapping the next distal barb. For illus- 

 tration I have chosen the fourth primary of an adult 

 Bewick's swan. In Fig. .\ the glistening area is left 

 unshaded ; in Fig. B a single barb is figured, with its 

 membrane ; Fig. C shows a portion of the rhachis with 

 the web cut across to show the barbs with their membranes 

 in section ; Fig. D gives in transverse section four barbs 

 with the curved membranes. This will be rendered clearer 

 by a reference to the fifth diagram, which figures the barbs 

 on a normal feather taken from a cormorant. In this 

 bird the membranes are wanting. 



So far as British birds are concerned, this feather 



element is present only in the ducks and their allies, where 

 it is always conspicuous ; in the four British grouse, where 

 it is again striking; and in the partridge and the pheasant. 

 In certain exotic game birds (Lophortyx, Tragopan, Gallus, 

 Catreus, &c.) it is quite absent, and this makes its uniform 

 constancy in the Anatidse all the more noteworthy. The 

 total absence of the structure in the feathers of Stegano- 

 podes, .Alcae, Pygopodes, Gavia;, and Tubinares suggests 

 that it is not essential to the feathers of water birds ; and 

 it is, moreover, as conspicuous on the feathers of the 

 Anatidae with terrestrial habits as it is on those of the 

 truly aquatic ducks. This leads one to look upon it as 

 vestigial of some earlier structure, and as such it might 

 be used for taxonomic purposes. 



The diagram of the primary shows the emarginations 

 or notches of the web that 'have been, and still are, used 

 in the classification of birds — Circus, for example. The 

 use or meaning of these notches has not been explained, 

 so I may be allowed to point out that the shapes of the 

 outer primaries are governed by the law of Avanzini. 

 Prof. Roy, in Newton's " Dictionary of Birds," has shown 



NO. 2133, VOL. 84] 



how this law affects the general shapes of the wing 

 feathers, but does not notice the notches. When the 

 wing is fully extended, with the primaries spread out like 

 the fingers of a hand, each feather must necessarily func- 

 tion as a separate wing or sail. As the position of the 

 rhachis cannot be altered, the web is trimmed away to the 

 necessary degree. The new feathers appear fully formed, 

 but, in many cases, Z have reason to suspect that the 

 notching is increased by the subsequent wear of the 

 adjacent feathers. Fkedk. J. Stubbs. 



stepney Borough Museum, E. 



An Interesting Donkey Hybrid. 



In his letter on the " Origin of the Domestic 

 ' Blotched ' Tabby Cat " (N.\ture, September 8, p. 29S), 

 .Mr. Vickers says, " after much diligent search I have been 

 unable to find a single instance in which complete segrega- 

 tion has taken place in respect of all specilic characters 

 when two well-defined species are crossed." Our know- 

 ledge of specific characters is too limited to make such a 

 claim provable if put forward ; but 1 have recently seen a 

 hybrid between two very distinct species which, at all 

 events, approaches that standard. This is a donkey 

 belonging to Sir Claud Alexander, Bart., which he tells 

 me was bred by Hagenbeck between a male dziggetai, or 

 -Mongolian donkey (Equus hemionns), and a female Nuliian 

 donkey {Equus asinus). Both these gentlemen are well 

 acquainted with the species in question, which, as every 

 zoologist knows, are very distinct forms. Yet, unless I 

 had been told that the animal was a hybrid, I should un- 

 hesitatingly have identified her as a pure-bred African 

 donlcey. Her colour is grey, her legs are strongly barred 

 with black, and she has a sharply defined black shoulder- 

 stripe and black mottling at the base of the long ears. 

 All these characters belong essentially to the .African, as 

 opposed to the Asiatic, species. 



In one point an approximation to the .Asiatic type is 

 shown. This is a widening of the spinal stripe towards 

 the croup, a feature which is certainly more marked than 

 in any .African donkey I have seen. Still, the stripe is 

 not nearly so wide as in the dziggetai ; and, seeing how 

 variable is the width of this stripe in quaggas belonging 

 to the same local race, I do not feel sure that its width 

 in the donkey in question is not an individual peculiarity 

 independent of inheritance. 



It is quite true, as Mr. Vickers says, although he 

 expresses the fact somewhat differently, that the progeny 

 of two distinct species usually combines the characters of 

 the parents in such a way as to be describable as inter- 

 mediate between them. The notorious case of Ward's 

 zebra may be quoted as an instance in point. Until its 

 history was known and its true nature ascertained, this 

 animal was regarded as a distinct species intermediate 

 between Chapman's quagga and the mountain zebra. It 

 is, in reality, as I have elsewhere shown, a hybrid between 

 the two ; and I cite the case here for comparison with 

 that of the donkeys. Whereas the two striped species of 

 Equus produced an " intermediate " when crossed, the two 

 nearly self-coloured species gave a very different result, 

 thus proving the impossibility of foretelling what the 

 progeny will be like when two well-defined species inter- 

 breed. It may be claimed, moreover, I think, that this 

 remarkable hybrid donkey weakens the force of Mr. 

 Vickers 's contention that the " blotched " and " striped " 

 tabby cats can hardly be representatives of distinct species 

 because their kittens are not intermediate between the 

 two types when crossed. R. I. PocoCK. 



Zoological Gardens, September i. 



British Marine Zoology. 



It is possible to have a considerable amount of sympathy 

 with Mr. S. Pace and also with his critic. Prof. MacBride, 

 and at the same time to differ from both on some points. 

 Mr. Pace aims high in both : — (i) his " bibliography of 

 all works dealing with the biology of the European seas," 

 and (2) his " exhaustive faunistic survey of the marine 

 life at one or more points on our coasts," and marine 

 biologists must wish him all possible success in his 

 venture ; but the doubt remains whether he has not under- 



