From this particular discussion I omit reference to those examples in which the permanently established types are obviously associated with special conditions of life. Where considerable climatic differences exist between localities, or when we pass from South to North, or from the plains into Alpine levels we often find that in correspondence with the change of climate there is a change in the characteristics of a species common to both. When I say "species" in such a connection I am obviously using the term in the inclusive sense. Some would prefer to say that in the two sets of conditions two _representative species_ exist. Whichever expression be preferred it is plain that such examples present another phase of the problem we have been just considering, and in them also we have an opportunity of observing the consequences of the overlap of two closely related types, but there are advantages in considering them separately. In the examples hitherto given, with the possible exception of the Papilios,[15] the two fixed types severally range over so extensive a region that it may fairly be supposed that in the different parts they are subject to considerable diversities of climate. There is no outstanding difference that we know distinguishing the habitats of the two forms; but in comparing Alpine with Lowland forms, or essentially northern with essentially southern forms we do know an external circumstance, temperature, that may reasonably be supposed to have an influence, direct or indirect, on the population. FOOTNOTES: [1] J. A. Allen, _The North American Species of the Genus Colaptes, Considered with Special Reference to the Relationships of C. auratus and C. cafer_. Bull. Am. Mus. Nat. Hist., IV, 1892. [2] For a case in which a red-headed female × a black-headed male gave a black-headed female and a red-headed male, see _Avian Mag._, N. S., IV, pp. 49 and 329 [3] The other variations of this bird are also interesting and important. The normal male has a red head and a red throat. The female has a red head and a white throat, but varieties of the female are known with a black head, thus again illustrating the change from black to red. It should be noted that this is not a mere retention of a juvenile character, but, as the birds mature, the red feathers come up, or as an exception, the black. There is also a western species, _ruber_, in which both sexes have a great extension of red, and are alike. The male of _nuchalis_ intergrades with this type, but the female does not. [4] Dr. W. Brewster, for example, has a remarkable specimen of the Teal (_Nettion carolinense_) with a white collar strongly developed at the front and sides of the neck, in a place where the normal has no such mark.