{381} _Nutria_ is the Spanish for otter, and is now a synonym for _Lutra_. The otter on the Atlantic coast is distinguished by minute differences from the Pacific species. Both forms are said to take to the sea. In fact the case presents no especial difficulties. {382} In _Origin_, Ed. i. p. 394, vi. p. 548, bats are mentioned as an explicable exception to this statement. {383} This reference is doubtless to _Mydaus_, a badger-like animal from the mountains of Java and Sumatra (Wallace, _Geographical Distribution_, ii. p. 199). The instance does not occur in the _Origin_ but the author remarks (_Origin_, Ed. i. p. 376, vi. p. 527) that cases, strictly analogous to the distribution of plants, occur among terrestrial mammals. {384} See _Origin_, Ed. i. p. 313, vi. p. 454. _On the number of species, and of the classes to which they belong in different regions._ The last fact in geographical distribution, which, as far as I can see, in any way concerns the origin of species, relates to the absolute number and nature of the organic beings inhabiting different tracts of land. Although every species is admirably adapted (but not necessarily better adapted than every other species, as we have seen in the great increase of introduced species) to the country and station it frequents; yet it has been shown that the entire difference between the species in distant countries cannot possibly be explained by the difference of the physical conditions of these countries. In the same manner, I believe, neither the number of the species, nor the nature of the great classes to which they belong, can possibly in all cases be explained by the conditions of their country. New Zealand{385}, a linear island stretching over about 700 miles of latitude, with forests, marshes, plains and mountains reaching to the limits of eternal snow, has far more diversified habitats than an equal area at the Cape of Good Hope; and yet, I believe, at the Cape of Good Hope there are, of phanerogamic plants, from five to ten times the number of species as in all New Zealand. Why on the theory of absolute creations should this large and diversified island only have from 400 to 500 (? Dieffenbach) phanerogamic plants? and why should the Cape of Good Hope, characterised by the uniformity of its scenery, swarm with more species of plants than probably any other quarter of the world? Why on the ordinary theory should the Galapagos Islands abound with terrestrial reptiles? and why should many equal-sized islands in the Pacific be without a single one{386} or with only one or two species? Why should the great island of New Zealand be without one mammiferous quadruped except the mouse, and that was probably introduced with the aborigines? Why should not one island (it can be shown, I think, that the mammifers of Mauritius and St Iago have all been introduced) in the open ocean possess a mammiferous