It was probably owing to the separation of North from South America, at that period, that the complete extinction of the Proboscidea, Equidæ, Rhinocerotidæ, and other Ungulata was there effected. These animals were probably driven south by the cold, and, not having a large southern continent to retreat to, were completely exterminated. The existing Tapiridæ are the only descendants amongst the Perissodactyla that exist in the Neotropical Region at the present day, none being now found in the Nearctic Region.
Amongst insects, Mr. Wallace has wisely confined his attention to the groups best known, as the Rhopalocerous Lepidoptera and the more showy amongst the Coleoptera, because these alone have been sufficiently studied to afford adequate material for generalization. Mr. Wallace has a singular power of suggesting an explanation of a difficulty, and this is well shown in his observations on the occurrence of numerous insects in South Temperate America which belong to genera found otherwise in north temperate regions only. Amongst the diurnal Lepidoptera, for instance, species of the genera Hipparchia, Argynnis, and Colias, and several genera allied to Erebia, are found in South Temperate America, and form a sufficiently remarkable group of northern forms, to render an explanation of their origin necessary. Mr. Wallace, indeed, admits that both in diurnal Lepidoptera and in Carabidæ, the northern element is fully equal to the tropical, if it does not preponderate over it. The whole of his argument is too long to be here extracted, but he points out that the great mass of neotropical butterflies are forest species, and for countless ages have been developed in a forest-clad tropical country. The north temperate butterflies, on the other hand, are for the most part species frequenting the open country, haunting pastures, mountains, and plains, and often wandering over an extensive area. These would find in the higher slopes of the mountains vegetation and conditions suited to them, and would occupy such stations in less time than would be required to adapt and modify the forest-haunting groups found in the American lowlands. It should not be forgotten, also, that along the higher regions of the Andes there is an almost continuous temperate region, which would provide for the animals of Northern Temperate America a district along which they could pass through Tropical America into the cooler regions of the south.
