I50 DIMORPHISM 



numbers than the ordinary form, Tringa alpina, is a smaller one 

 which has received the specific name of T. schinzi, while in the 

 New World our common T. alpina is comparatively scarce, and a 

 larger form, the T. americana of some authors, is the more abundant. 

 It is difficult to determine at present whether this is a case of local 

 races or one of Dimorphism — though here Trimorphism might be 

 the more proper word. 



Among birds examples of sexual Dimorphism are so numerous 

 as to make it almost the rule. Yet, as already stated and as is 

 widely known, this kind of Dimorphism manifests itself in very 

 many ways — the commonest being that of general coloration, in- 

 stances of which will occur to every one; but apart from that the 

 coloration of particular parts is scarcely less often divergent in the 

 two sexes, while diff'erences of the form or development of certain 

 portions of the plumage are also very abundant, as witness the 

 occipital plumes in the male of many birds, while the extraordinary 

 elongation of the feathers of the lower back in the Peacock, of 

 those on the side of the breast in the Bird of Paradise, or of the 

 tail in the BLACKCOCK are notorious. Passing to characters which 

 may be of greater signifiicance, we have spurs on the metatarsus or 

 near the wrist, the former only among the Gallinse, but the latter 

 found in birds of several groups that are not nearly allied. These 

 are generally and justly admitted to be weapons, and hardly less 

 effective are the knobs which occupy the like position in other 

 forms, those of the male Fezophaps being perhaps the most remark- 

 able. Sexual Dimorphism of the Bill has been already noticed, 

 and it extends in various ways to the head, wattles, frontal plates, 

 protuberances that are permanent or only temporarily erectile, which 

 are far too numerous to mention ; but other much more special 

 peculiarities are the sublingual bag of Bizmra lobata, the seasonal 

 pouch of the Bustard, and the inflatable sacs of the Prairie-foAvls 

 (Grouse), while the convolutions and enlargements of the trachea 

 in many birds (e.g. Manucode) though not externally visible pro- 

 duce an audible sexual Dimorphism. 



Sexual Dimorphism in size is also manifested among birds — 

 and this in both directions. To ourselves it may seem natural that 

 the male should be the stronger and therefore the bigger sex, and 

 among Mammals he generally is ; but in Birds this is by no means 

 so much the rule, the cock being very considerably larger than the 

 hen only in certain Gallinaceous and Eatite groups, most of which 

 are polygamous, and hence a possible explanation may be afforded. 

 On the other hand, though a case in which the female is larger 

 than the male is hardly to be found among Mammals, instances 

 occur among Reptiles (notably in Tortoises and Snakes) and veiy 

 frequently among Amphibians and Fishes. Among Birds it is 

 almost universal with the Accipitres, and obtains in several of the 



