364 Universitij of California PiMications in Zoology [Vol. 11 



insufficient nutrition. That the change in type of structure i.s 

 due to the latter and not to the change in rate of growth is 

 further indicated by the invariable way in which the downy 

 structure begins to be developed at the point farthest from the 

 source of nutrition, i.e., at the tips of the barbs, and gradually 

 approaches the shaft. Were the change due to a change in rate 

 of growth, all the barbs would be affected at the same level and 

 the line of demarcation between the downy and differentiated 

 portion would be parallel to a horizontal fault bar, i.e., almost 

 at right angles to the shaft. 



In the greater upper coverts again we are struck by the 

 nicety of the details of structure exhibited. In this case the 

 most noticeable special adjustments are (1) the sigmoid curve 

 of the shaft, instrumental in bringing the feathers to lie between 

 and not squarely above the remiges, and (2) the depth of the 

 shaft as compared with the width, rendering it more resistant 

 to vertical stresses, and therefore firmer. The greater under 

 coverts are peculiar, as noted previously, in the fact that they 

 lie with their reverse side exposed, thus differing from all the 

 other feathers of the body. The explanation for this .state of 

 affairs offered by Wray (1887) is very suggestive, namely, that 

 this series of coverts represents a series of feathers which at 

 some stage in the phylogeny was equal to or larger than the 

 next row, or true remiges, and when the latter outdistanced them 

 and covered them up, they remained in the same relative position, 

 but functioned as coverts. They, as well as the upper coverts, 

 have the same overlap as the remiges, while all the other coverts 

 have a reversed overlap. The absence of the aftershaft is an 

 almost necessary corollary of the reversed position of the feather. 

 In these under coverts there is a tendency for the barbules to 

 become weaker and less specialized in structure, since they no 

 longer serve such an important function, and are not exposed to 

 such severe stresses and strains. The greatly elongated tips of 

 the barbules tend to produce an even and less penetrable surface. 



Some of the most interesting adaptations to function are the 

 various ways in which the edge of the wing is made adequate 

 in different places, as described under the heading "Alula." 

 The latter feathers are of morphological interest, being a series 



