150 DIMORPHISM 
numbers than the ordinary form, Zringa alpina, is a smaller one 
which has received the specific name of TZ. schinzi, while in the 
New World our common 7’ alpina is comparatively scarce, and a 
larger form, the 7. 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 differences 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 BrrpD OF PARADISE, or of the 
tail in the BLACKCOCK are notorious. Passing to characters which 
may be of greater significance, we have spurs on the metatarsus or 
near the wrist, the former only among the Gallinz, 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 Pezophaps being perhaps the most remark- 
able. Sexual Dimorphism of the BiLu 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 biziwra lobata, the seasonal 
pouch of the BusTArD, and the inflatable sacs of the Prairie-fowls 
(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 
su much the rule, the cock being very considerably larger than the 
hen only in certain Gallinaceous and Ratite 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 very 
frequently among Amphibians and Fishes. Among Birds it is 
almost universal with the ACCIPITRES, and obtains in several of the 
