NESTLING FEATHERS OF THE MALLARD. 639 



long, dense wooll}' covering, so dense that it is difficult to expose 

 the skin." In course of time the protoptile coat is shed and the 

 mesoptile coat is pushed from the skin by the tr\ie feathers. 



The long fur-like mesoptile coat of the King Penguin suggests 

 that during the Ice Age, which succeeded the period of desic- 

 cation, the evolution of true feathers was ai-rested, and that the 

 mesoptiles cannot be regarded as representing a stage in the 

 evolution of true feathers. Though to the naked eye the meso- 

 ptile barbules seem to be identical with the barbules forming the 

 protoptiles, they are seen to differ when examined under the 

 microscope — instead of being nearly straight they are spirally 

 twisted at their origin. In the case of King Penguin chicks the 

 mesoptiles differ from the protoptiles more than in the Adelie 

 chicks, and, as already pointed out, the mesoptiles of the Ringed 

 Penguin decidedly differ from both the protoptiles and the 

 metaptiles. In all the three Penguins mentioned the first true 

 feathers differ from the prepennfe which precede them. Hence, 

 though the prepennse and all the true feathers which succeed 

 them must be regarded as links in a chain, it should be borne in 

 mind that the first link diffei'S both in function and structure 

 from the second, and the second from the third. Further, it 

 should be borne in mind that there are good reasons for believing 

 that, but for the advent of an Ice Age, the protoptiles at a 

 compai-atively early period would have been succeeded by true 

 feathers, and that probably long before Archfeopteryx made its 

 appearance on the scene some millions of years ago, steps would 

 have been taken in most birds to suppress the fur-like coat of 

 mesoptiles, now apparently only well developed in Penguins. 



In conclusion, it may be stated that a study of the develop- 

 ment of birds suggests : (1) that their coat, to start with, consisted 

 of vai"ious kinds of simple filaments; (2) that in course of time 

 the hollow epidermic cone in each filament, in some incompre- 

 hensible way, was converted into a protoptile or preplumula : 

 (3) that the protoptiles in some cases soon acquired the chief 

 chai'acteristics of true feathers ; (4) that for a time progress was 

 arrested in order to provide birds (or most of them) with a fur- 

 like (mesoptile) coat, by way of giving them a chance of surviving 

 dviring the cold phases of an Ice Age : (5) that as the climate 

 improved the mesoptile coat was in many cases superseded by a 

 coat of true feathers ; and (6) that by the specialization of feathers 

 along the posterior margin of the hands and forearms and the sides 

 of the tail, birds of the Archseopteryx type were eventually evolved 

 capa.ble of flight, or at least of gliding easily from tree to tree. 



Text-figures 1, 2, 8, 9, 12 are from drawings by Miss Augusta 

 Lamont, B.Sc, text-figures 3-7 and 10, 11, 13, 14 are from 

 drawings by the author, all the other figures are from photographs 

 by Mr. John Chisholm. 



Towards defraying the cost of the drawings and photographs 

 a grant was obtained from the Moray Fund of the University of 

 Proc. Zool. Soc— 1 92 1 . N"o. XLIII. 43 



