NESTHXG FEATHERS OF THE MALLARD. 623 



domestic ducks. In cross-bred, as in Mallard ducklings, the 

 feathers at each side of the true tail-quills approach in structure 

 the wing-quill mesoptiles. In Eider ducklings the wing-quill, as 

 well as the tail-quill, mesoptiles are either absent or represented 

 by miniite vestiges, hence the Eider supports the view that in 

 the Anseres the mesoptiles no longer count in the struggle for 

 existence. 



(5) The Prepennce of the Trunk. 



About the prepennfe which precede the feathers of the head, 

 neck, trunk, and hind limbs, very little need be said. In many 

 cases the protoptiles closely resemble, and may be as large as, 

 the wing-quill protoptiles, but they are sometimes so small and 

 poorly developed that it is difficult to distinguish them from pre- 

 plumulaj. The protoptile from the dorsal track of a thirty-eight 

 days' Mallard duckling represented in PI. I. fig. 1 is only half the 

 size of the wing-quill protoptile in PI. II. fig. 8 ; but some of the 

 protoptiles of the wing-coverts are smaller than the majority of 

 the preplumulae. Usually the protoptiles of the trunk are shed 

 about the same time as the wing-quill protoptiles, but in a small 

 chequered area at each side of the body near the tail well- 

 developed protoptiles are still present at the end of the eleventh' 

 week. Though mesoptiles precede some of the trunk pennse, 

 they are never as well developed as the wiug-quill mesoptiles. 



The examination of the mesoptiles of ducks and penguins lends 

 support to the view that the mesoptile coat was specially acquired 

 to meet a marked change of climate — a change perhaps from cold 

 dry to cold damp conditions — and that in most cases as the 

 climate improved the fur-like second coat was more or less com- 

 pletely suppressed, and a new coat, made up of feathers con- 

 structed on the piotoptile rather than the mesoptile plan, was 

 subsequently evolved. It is conceivable that some of our modern 

 birds, e.^r.jtheEmu and Cassowary, are descended from ancestors 

 which never acquired a mesoptile plumage. 



2. The Prephcmuke and Plunudce. 

 (1) The Prephcviulce. 



In many birds the nestling coat sooner or later consists of pre- 

 plumulfe as well as prepennpe. In the Gannet the preplumula; 

 appear before the prepennee, and grow so rapidly that even when 

 the prepennse are well developed they are difficult to fii:id ; but 

 in the Mallard in most regions the preplumulae are hidden by a 

 relatively dense coat of protoptiles. Though the pre2:)lumuke 

 papillae in the Mallard fippear three or four days later than the 

 protoptile papillte, the majority of the prepiumulge are ready to 

 escape from their filanienls ns soon as the duckliug is hatched. 



The relative size of the protoptile and preplumuhe filaments of 

 the Mallard is indicated in PI. III. fig. 9 and in text-fig. 10. The 

 difference in size of the protoptiles, preplumulae, and prefiloplumse 



Proc. ZooL. Soc— 1921,No. XLII. 42 



