AVES. 



353 



closing the external part, which is of a conical 

 form ; the latter extends from the base of the 

 bulb, and gradually diminishes to a point 

 where the shaft is completed and the barbs 

 begin to expand. Its office is to deposit the 

 pith within the shaft, and it is absorbed in 

 proportion as this is effected. The internal 

 part or case also commences at the base of the 

 bulb, and adheres closely to the cone, with 

 which, indeed, its substance is continuous; it 

 increases in thickness as the cone diminishes, 

 its margins are beautifully scolloped or crenate, 

 and the crenations are lodged in the interspaces 

 of the oblique laminae or moulds, and deposit 

 in them the material of the vane. The horny 

 sides of the shaft are lodged and formed in the 

 grooves between the external and internal parts 

 of the bulb, and correspond in degree of 

 formation to the depths of those grooves, and 

 being progressively brought into contact from 

 above downwards, the shaft is thus completed, 

 leaving the longitudinal line at the internal 

 side. When all the grooves, (wherein are 

 formed the barbs, and the portion of the shaft 

 which carries them) are filled by the horny 

 matter, and the barbed part of the feather is 

 finished, this horny matter lastly expands uni- 

 formly around the medulla, and forms the quill 

 of the feather. 



When the quill of the feather has acquired 

 the due consistence, the internal medulla be- 

 comes dried up, and is resolved, as before, into 

 membranous cones arranged one upon the 

 other ; but these latter never pass out, for the 

 quill, which is now hardened and closed by 

 the shaft at the opposite extremity to the lower 

 umbilicus, will not permit their egress ; they 

 remain, therefore, inclosed, and constitute the 

 light dry pith which is found in the interior of 

 the quill. The last remains of the bulb are seen 

 in the ligament which passes from the pith 

 through the lower opening of the quill and 

 attaches it to the skin. 



Cuvier has justly observed that notwith- 

 standing the complexity of the process just de- 

 scribed, the formation of a feather differs only 

 from that of a tooth in the nature of the substance 

 which is deposited between the two tunics 

 which constitute its mould; but a tooth takes 

 many years to be perfected, and there are but 

 two series produced in one part of the jaw, and 

 only one in the other, in any warm-blooded 

 animal. Feathers, on the other hand, are de- 

 veloped in the course of some days ; they 

 attain a length of from one to two feet or more 

 in many birds, and they are almost all re- 

 newed every year, in many species even twice 

 a year. It may be conceived, then, how much 

 vital energy the organization of birds must 

 exercise, and how many dangers must accom- 

 pany so critical a period as that of the moult. 



The plumage is commonly changed several 

 times before it attains that state which is re- 

 garded as characteristic of the adult bird. 

 The time required for this varies from one to 

 five years, and several birds rear a progeny 

 before they acquire the plumage of maturity. 



When the male bird assumes a. vestment 

 VOL. r. 



differing in colour from the female, the young 

 birds of both sexes resemble the latter in their 

 first plumage ; but when the adult male and 

 female are of the same colour, the young have 

 then a plumage peculiar to themselves. Mr. 

 Yarrell states a third law in addition to the 

 preceding, viz. that whenever adult birds as- 

 sume a plumage during the breeding season 

 decidedly different in colour from that which 

 they bear in winter, the young birds have a 

 plumage intermediate in the general tone of its 

 colour compared with the two periodical states 

 of the parent birds, and bearing also indica- 

 tions of the colours to be afterwards attained 

 at either period. 



" There are three modes," the same author 

 observes, " by which changes in the appearance 

 of the plumage of birds are produced : 



" By the feather itself becoming altered in 

 colour. 



" By the bird's obtaining a certain number 

 of new feathers without shedding any of the 

 old ones ; and 



" By an entire or partial moulting, at which 

 old feathers are thrown off and new ones pro- 

 duced in their places. 



" The first two of these changes are ob- 

 served in adult birds at the end of spring, in- 

 dicating the approach of the breeding season ; 

 the third change is partial in spring and entire 

 in autumn. 



" A fourth tnode may be noticed, though 

 its effects are limited. It is observable in 

 spring, as the breeding season approaches, by 

 the wearing off of the lengthened lighter- 

 coloured tips of the barbs of the feathers on 

 the body, by which the brighter tints of the 

 plumage underneath are exposed, as was no- 

 ticed by Sir William Jardine and Mr. Blyth. 

 The effect is most conspicuous in the Buntings, 

 Finches, and Warblers"* 



The experiments detailed in the Memoir 

 above quoted, some of which we witnessed, 

 prove incontestably, that notwithstanding the 

 extravascular nature of feathers, they are 

 subject to influences, apparently of a vital 

 nature, which occasion a change of colour in 

 them after they are completely formed. In 

 yearling birds the winter plumage which suc- 

 ceeds the autumnal moult gradually assumes 

 the brighter tints characteristic of the adult 

 without a change of feather. The new colour 

 commences generally at that part of the web 

 nearest the body of the bird, and gradually 

 extends outwards till it pervades the whole 

 feather. 



Organs of generation. The few varieties 

 of structure which these organs present in the 

 Class of Birds, are principally met with in 

 those of the male, which we shall first de- 

 scribe. 



The male organs of generation exhibit all 

 the essential characteristics of the oviparous 

 type of structure. The testes are situated high 

 up in the abdomen, whence they never descend 

 into an external scrotum. The intromittent 



* Yarrell, Zool. Trans, i. p. 13. 



2 A 



