Wing, 11.50; tail, 6.60; culmen, .70; tarsus, 1.90; middle toe, 1.65. Above uniform dark brown, with a faint plumbeous cast, the feathers without trace of light or rusty edges; outer web of tail-feathers without trace of light spots. _Hab._ Mexico … var. _mexicanus_.[50] Wing, 13.60–14.30; tail, 8.25–9.00; culmen, .80–.87; tarsus, 1.85–1.90; middle toe, 1.85–1.90. Colors similar to the last; entire auriculars white; mustache narrow and conspicuous. _Hab._ Southern Asia … var. _jugger_.[51] The only point of difference in the external anatomy between the Lanner Falcons and Gerfalcons consists in the different degree of feathering on the upper part of the tarsus; this is much denser and extends farther down and more around the posterior face in the Gerfalcons, but they, being inhabitants of a very northern latitude, need this protection against the rigor of the climate. These slight specific differences are illustrated by the figures on page 1430. The same difference is observable in many birds whose habitat extends through a great range of latitude, as, for instance, the _Pediocætes phasianellus_, the northern race of which has the feathers covering the base of the toes so long as to reach beyond the claws and nearly conceal them, while in the southern form (var. _columbianus_) the toes are almost completely naked. My determination of the number and character of the geographical races of _F. gyrfalco_ is the result of a very careful critical examination of over sixty specimens, aided by the important conclusions of Mr. Hancock (Annals and Magazine of Natural History, 2d ser., XIII, 110; London, 1834), Schlegel (_Falcones_, Muséum d’Histoire Naturelle des Pays-Bas, 1862), Pelzeln (Uebersicht der Geier und Falken der Kaiserlichen ornithologischen Sammlung, April, 1863), and Alfred Newton (History of British Birds, revised ed., part 1, June, 1871, pp. 36–52, and Proc. Acad. Nat. Sc. Philadelphia, July, 1871, pp. 94, 95), in their important papers bearing upon this subject, which, though they each express the peculiar individual views of the writer, together clear up pretty satisfactorily the problem of the number, character, and habitats of the several races, as well as the different phases of variation to which each is subject. [Illustration: =43139=, ♀. ½ _Falco sacer._] [Illustration: =43139=, ♀. ¼] [Illustration: =5482=, ♀. ½ _Falco polyagrus._] [Illustration: =43139=, ♀. ½] [Illustration: =5482=, ♀. ½]