EXTERNAL PAliTS OF BIIiDS. ^ TOPOGRAPHY. 91 



vented from acquiring tlie briglit colors of the males, by the destruction which they Huffered during incubation. 

 There is no evidence that it is poKsible by natural selection to convert one form of transndssion into another. But 

 there would not be tlie least difficulty in rendering a female dull-coh)red, the male being still kept I>right-color<-(i , 

 by the selection by successive variations, which were from the tirst limited in tlieir traiismission to tlie same nex 

 Whether the females of many species have actually been thus modified, must ;it present lemain doubtful. When, 

 through the law of the equal transmission of characters to both sexes, the females were rendered as conspicuously 

 colored as the males, their instincts appear often to have been modified so that they were led to build domed or 

 concealed nests. 



" In one small and curious class of cases the characters and habits of the two sexes have been completely 

 transposed, for the females are larger, stronger, more vociferous and brighter colored than the males. They have, 

 also, become so quarrelsome that they often light together fur the possession of tht; males, like the males of other 

 pugnacious species for the possession of the females. If, as seems i)robable, such females halutually drive away 

 their rivals, and by the display of their bright colors or other charms endeavour to attract the males, we can under- 

 stand how it is that they have gradually been rendered, by sexual selection ajid sexually-limited transmission, 

 more beautiful than the males — the latter being left unmodified or only slightly moditIe<l. 



" Whenever the law of inheritance at corresponding ages prevails, but not that of sexually-limited trans- 

 mission, then if the iiarents vary late in life — and we know that this constsintly occurs with our ijoultrj, 

 and occasionally with other birds — the young will be left unafE'ected, whilst tke adults of both sexes will bti; 

 modilled. If both these laws of inheritance prevail and either sex varies late in life, that sex alone will be 

 modified, the other sex and the young being unaffected. When variations in brightness or in other consi)icuous 

 characters occur early in life, as no doubt often liapjiens, they will not be acted on thron^^h sexu;il selection until 

 the j)eriod of rei)roduction arrives; consequently if dangerous to the young, they will bo t.-hmiuated through 

 natural selection. Thus we can understand how it is that variations arising late in life ha\'e so often been pre- 

 served for the ornamentation of the males ; the females and the young being left almost unaftecied, and therefore 

 like e'ach other. With species having a distinct summer and winter plumage, the miUes of which either resemble 

 or ditter from the females during both seasons or during the summer alone, the degrees and kinds of resemblance 

 between the young and the old are exceedingly complex; and this complexity ap|)arently depends on characters, 

 tirst acquired by the males, being transmitted in various ways, as limited by age, sex, and season. 



" As the young of so many species have been but little modified in color and other ornaments, we are enabled 

 to form some judgment with respect to the plumage of their early i)rogenitors ; and we may infer that the beauty 

 of our existing species, if we look to the whole class, has been largely increased since that period, of whic;h the 

 plumage gives us an indistinct record. Many birds, especially those which live much on the ground, have undoubt- 

 edly been obscurely colored for the sake of protection. In some instances the upper exposed surface of the plumage 

 has been thus colored in both sexes, whilst the lower surface in the males alone has been variously ornamented 

 through sexual selection. Finally, from the facts given in these four chapters [jjp. 35S-499 of the work in citation], 

 we may conclude that weapons for battle, organs for producing sound, orniiments of many kinds, bright and con- 

 spicuous colors, have generally been acquired by the males through variation and sexual selection, and have been 

 transmitted in various ways according to the several laws of inheritance — the female and the young being left 

 comparatively but little modified." 



h. The Topography of Birds. 



The Contour of a Bird with the feathers on is spin (Ue- shaped; or fusiform {Lat. 

 fttsiis, a spiudle), tapering- at both ends ; it r(^presents two cones jt)ined base to base at the middle 

 or greatest girth of the body, tapering in front to the tip of the bill, beliind to the end <,)f tlie 

 taih The obvious design is easiest cleavage of air in front, and least drag or wash beliind, in 

 the act of flying. This shape is largely produced by the lay of the plumage ; a naked bird pre- 

 sents sevei'al prominences and depressions, this irregular contour being reducible, in general 

 terms, to two spindles or double cones. The head tapers to a point in front, at the tip of thr 

 bill, and contracts behind, toward the middle of the neck, hi consequence of dnninution in 

 bulk of the muscles by which it is slung on the neck ; which last is somewhat contracted or 

 hour-glass shaped near the middle, swelling where it is slimg to the body. The body is largest 

 in front and tapers to the tail. The 



Centre of Gravity is admirably preserved beneath the centre of the body, and opposite 

 the points where it is supported by the wings. The enormous breast-muscles of a bird are 

 among its heawst parts, sometimes weighing, to speak roundly, as much as one-sixth of the 

 whcde bird. Now these are they that effect all the movements ftf the wings at the shoulder- 

 joints, lifting as well as lowering the -v^dngs. Did these pectoral muscles pull straight, the 

 lifters would have to be above the shoulder-joint ; but they all lie below it, and the lifters 



