96 GENERAL ORNITHOLOGY. 



alone, the more important result wiU have been an Increased tendency to vary or to present more strongly-marked indi- 

 vidual differences ; and such differences will have afforded an excellent ground-work for the action of sexual selection. 



" The laws of inlieritance, irrespectively of selection, appear to have determined whether the characters acquired 

 by the males for the sake of ornament, for producing various sounds, and for fighting together, have been transmitted to 

 the males alone or to both sexes, either permanently, or periodically during certain seasons of the year. Why various 

 characters should have been transmitted sometimes in one way and sometimes in another, is not in most cases known ; 

 but the period of variability seems often to have been the determining cause. When tlie two sexes have inlierited all 

 characters in common, they necessarily resemble each other ; but as the successive variations may be differently trans- 

 mitted, every possible gradation may be found, even within the same genus, from the closest similarity to the widest dis- 

 similarity between the sexes. With many closely-allied species, following nearly the same habits of life, the males have 

 come to differ from each other chiefly through the action of sexual selection ; whilst the females have come to differ 

 cliiefly from partaking more or less of the characters thus acquired by the males. The effects, moreover, of the definite 

 action of the conditions of life, will not have been masked in the females, as in the males, by the accumulation through 

 sexual selection of strongly-pronounced colors and other ornaments. The individuals of both sexes, however affected, 

 will have been kept at each successive period nearly uniform by the free intercrossing of many individuals. 



" With species, in which the sexes differ in color, it is possible or probable that some of the successive variations 

 often tended to be transmitted equally to both sexes ; but that when this occurred the females were prevented from ac- 

 quiring the bright colors of the males, by the destruction which tliey suffered during incubation. There is 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 spe- 

 cies have actually been thus modified, must at present remain doubtful. When, through the law of the equal transmis- 

 sion 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 trans- 

 posed, for the females are larger, stronger, more vociferous and brighter colored than the males. They have, also, be- 

 come 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 endeavor to attract the males, we can understand how it is that they 

 have 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 transmission, 

 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 un- 

 affected. When variations in brightness or in other conspicuous 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 preserved 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 Uttle 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 undoubtedly 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. 358-499 of the work in citation], we may conclude that weapons 

 for battle, organs for producing sound, ornaments of many kinds, bright and conspicuous 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." 



l. Topography of Birds. 



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

 ftisus, a spindle), tapering at botli 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 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 the 



