EXTEMNAL PARTS OF BIBDS. — TOPOGBAPHY. 91 



vented from acquiring the bright colors of the males, by the destruction which they suffered during incubation. 

 There i8 no evidence that it is possible by natural selection to convert one form of transmission into another. But 

 there would not be the least difficulty in rendering a female dull-colored, the male being still kept bright-colored, 

 by the selection by successive variations, which were from the first limited in their transmission to the same sex. 

 Whether the females of many species have actually been thus modified, must at present remain doubtful. "When, 

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

 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 fight together for the possession of the males, like the males of other 

 pugnacious species for the possession of the females. If, as seems probable, such females habitually 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 h,ave gradually been rendered, by sexual selection and sexually-limited transmission, 

 more beautiful than the males— the latter being left unmodified or only slightly modified. 



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

 mission, then if the parents vary late in life — and we know that this constantly occurs with our poultry, 

 and occasionally with other birds — the young will be left unaffected, whilst the adults of both sexes will be 

 modified. 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 conspicuoua 

 characters occur early in life, as no doubt often happens, they will not be acted on through sexual selection until 

 the period of reproduction arrives; consequently if dangerous to the young, they will be eliminated through 

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

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

 like each other. With species having a distinct summer and winter plumage, the males of which either resemble 

 or differ 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 apparently depends on characters, 

 first 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 progenitors ; 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 which the 

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

 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 [pp. 3B8-499 of the work in citation], 

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

 spicuous colors, have generally beer, 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." 



6. The Topography op Bieds. 



The Contour of a, Bird with the feathers on is spindle-shaped, or fusiform (Lat. 

 fiisus, a spindle), tapering at both ends; it represents two cones joined base to base at the middle 

 or greatest girth of the body, tapering in front to the tip of the bill, behind to the end of the 

 tail. The obvious design is easiest cleavage of air in front, and least drag or wash behind, in 

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

 sents several prcjminences and depressions, this iiTegular contour being reducible, in general 

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

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

 bulk of the muscles by which it is slung on the neck ; which la,st is somewhat contracted or 

 hour-glass shaped near the middle, swelling where it is slung 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 heaviest parts, sometimes weighing, to speak roundly, as much as one-sixth of the 

 whole bird. Now these are they that eflect all the movements of the wings at the shoulder- 

 joints, lifting as well as lowering the wings. Did these pectoral muscles puU straight, the 

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



