26 THE BIOLOGY OF BIRDS 



with, consisted of various kinds of simple filaments ; (2) that 

 in course of time the hollow epidermic cone in each filament, 

 in some incomprehensible way, was converted into a 

 protoptile or preplumula ; (3) that the protoptiles in some 

 cases soon acquired the chief characteristics 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 during 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 the mesoptile coat was in many cases superseded by a coat 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 specialisation of 

 feathers along the posterior margin of the hands and fore- 

 arms and the sides of the tail, birds of the Archaeopteryx 

 type were eventually evolved capable of flight, or at least 

 of gliding easily from tree to tree." 



Development o£ a Feather. — The early development 

 of a feather is in a general way like that of a scale. A 

 papilla projects on the delicate skin of the embryo (about 

 the 6th day in the chick) ; this papilla has a core of dermis 

 (mesodermic), which feeds the growing feather, and a 

 covering of several layers of epidermis, an unimportant 

 delicate epitrichium, a stratum corneum (forming a lining 

 to the feather follicle) and the inner stratum Malpighii 

 (forming the final feather and its transitory sheath). The 

 papilla becomes a backward directed cone and sinks into a 

 moat or feather-follicle which is largely due to the ingrowing 

 of the Malpighian layer (Fig. 2, IV). 



In the centre of the young papilla there is the nutritive 

 pulp, around this a mantle of Malpighian cells, outside this 

 the inturned stratum corneum, and outside this the dermis 

 or under-skin into which the follicle has grown down. 



But the Malpighian mantle differentiates into three 

 layers — (i) the innermost delicate transparent sheath of 

 the pulp, afterwards seen as the " caps " in the quill ; 

 (2) the longitudinal rows of cornified cells which become 

 the barbs and coalesce in a cylindrical tube or calamus at 

 the base ; and (3) an external sheath which afterwards 

 splits and peels off, Hberating the barbs (Fig. 2, V). 



