the two forms in the places where they co-exist is maintained by the seasonal isolation. Just as the consequences of isolation are to be seen in the most different forms of life so may they also affect the most diverse features of organisation, such as size, colour, sculpture, shape, or number of parts. In the Sloth (_Choloepus_) the geographical races differ in the number of cervical vertebrae--or in other words, in the distribution of vertebral differentiation. The geographical races of _Cistudo_ differ in the number of claws and phalanges.[2] In Shetland, the males of _Hepialus humuli_ (the Ghost Moth) are not sharply differentiated in colour from the females, as they are elsewhere, but in varying degrees resemble them.[3] No such males are found in other localities, and even in the other Scottish islands they are normal. In the island of Waigiu the converse phenomenon has been observed in _Phalanger maculatus_. Generally the male is spotted with white, and the female is without spots, but in Waigiu the females are spotted like the males.[4] The following striking illustration was pointed out to me by Dr. W. D. Miller. _Euphonia elegantissima_ as it occurs in Mexico and Central America has the two sexes very distinct from each other. The male has the lower parts orange and the upper parts a dark indigo blue, with a bright turquoise-blue head and neck. The female, except for the head, is of a bright olive green. A form in which the sexes are similarly differentiated exists in Porto Rico and is known as _E. Sclateri_. But in many of the other West Indian islands the representative "species" (_E. flavifrons_) has the two sexes closely resembling the _female_ of _E. elegantissima_. This form is found in Antigua, Barbados, St. Vincent, and Guadeloupe, from which localities the British Museum has specimens. All three so-called species are very much alike otherwise. In the genus _Pyrrhulagra_ (_Loxigilla_) to which Mr. Outram Bangs called my attention, several distinct and alternative possibilities occur. The genus has many local species occurring on the various West Indian islands. These species are characterized by differences in size, colour, and the shape of the bill. The colours have a narrow range, being black or greyish, with or without chestnut marks about the head and throat. In most of the islands the males are in general colour a full black, and the females are distinctly grey. They are thus found in San Domingo, Jamaica, Bahama, and most of the Lesser Antilles. In Porto Rico we meet the peculiarity that the hens are almost as black as the males (Ridgway describes the black of the hens as slightly less intense). This form is called _portoricensis_. A larger type, known as _grandis_, similarly coloured, inhabits St. Kitt's. Then, on the contrary, in Barbados, _both sexes_ are a dull blackish grey, like the hens of the Lesser Antilles in general.