NESTLING FKATHERS OB^ THE MALLARD. 637 



A series of links connect the relatively simple umbelliform 

 protoptiles of Pengnins with the highly-specialized protoptiles of 

 Dncks and Emus. There are good reasons for believing that the 

 warm-blooded progenitors of birds and mammals were evolved 

 out of reptile-like ancestors during a period when desert con- 

 ditions prevailed. How long this period lasted, and to what extent 

 feathers had been specialized when it came to an end it is impos- 

 sible to say, but judging by what happens during the development 

 of the wing-quills of the Mallard, it may be assumed the period of 

 aridity came to an end, and a glacial period had fairly set in long 

 befoi-e wing-quills and other true feathers were evolved. In 

 other words, before protoptiles had been modified to form meta- 

 ptiles and teleoptiles, progress in the evolution of true feathers was 



Text-figure 13. 



Section througli the filament of a Pigeon in which the middle epidermic layer has 

 been split longitudinally to form barbs, s., the thin sheath which disintegrates 

 immediately after hatching, h., one of the eight barbs formed by the splitting 

 of the hollow cone formed by the middle epidermic layer, p., the pulp con- 

 taining blood vessels, i. e., the dermis or true skin which extends during 

 development to the tip of the protoptile. After Davies. 



arrested owing to the necessity of providing birds with a fur- 

 like coat as capable of arresting the flow of heat from the skin as 

 the dense coat now worn by Polar Bears and other Arctic 

 mammals. 



The discovery by Dr. Eagle Clarke and Mr. Pyeraft of a 

 mesoptile coat in penguins has profoundly modified our views 

 about the plumage of birds. We do not yet know if the remote 

 ancestors of all our modern birds acquired a mesoptile coat — 

 probably in the case of the ancestors of the Ratitae this was 

 unnecessary — but the more the nestling feathei's are studied tlie 

 evidence of the existence of two coats of prepenn?e in the ancestors 

 of modern birds is increased. The fur-like mesoptile coat is 

 probably as well developed to-day in penguins as it was in their 

 remote ancestors of the Jui-assic Age ; relatively large mesoptiles 

 still occiir in ducks and geese, and I have found recently distinct 

 vestiges of a mesoptile coat in pigeons. How long birds, or most 



