152 THE INFANCY OF ANIMALS 



the preceding generation or the contour feathers soon 

 to follow, being of a pale rust-colour barred with 

 black. 



Furnished with this clue, one naturally began to examine 

 nestling down of all sorts of birds, and the surprising 

 fact soon came to light that this mesoptyle plumage 

 could be traced in quite a large proportion, though in 

 most cases it was of a degenerate character, only to be 

 distinguished from down feathers of the first generation 

 (protoptyles) when found in direct association therewith. 

 Herein we have another lesson in evolution associated 

 with degeneration. 



As a matter of fact these two phenomena are commonly 

 associated, but not always directly. To get at the bottom 

 of the matter of this degenerate mesoptyle down, then, it 

 is necessary to bear in mind the fact that, in the develop- 

 ment of adult plumages, there are growth pauses. One 

 generation of feathers, say that of the breeding plumage, 

 completes its growth, and the growing or formative tissue 

 at its base, as it were, dries up : when at the approach 

 of the autumn moult the activities of the formative tissue 

 begin again, the old feather is thrust out by the growth 

 of the new one at its base. But when one generation 

 of feathers rapidly succeeds another, it may happen, and 

 does happen in the case of these nestling downs, that 

 the growth of the mesoptyle begins before that of the 

 protoptyle is finished, so that the first is organically 

 connected with the second. 



In some cases there is a hardly perceptible growth 

 pause ; such as there is can be traced only by the micro- 

 scope, and then reveals itself in the absence of barbules 

 along a part of the continuous barb common to both 

 generations. The absence of barbules, in other words, 



