714 THE ORIGIN OF SPECIES 



4. The distribution of animals includes another group of facts 

 of which Darwin made much in his argument for the probabihty 

 of evolution. The origin of the distribution of animals, like all 

 other distributional problems, may be divided, in modern terms, 

 into the geographical and ecological aspects, the former paying 

 attention to mere distance as an isolating factor, the latter to 

 special quahties of the environment. It is obvious to any traveller 

 that on the whole the farther apart two places are, the less similar 

 are their faunas. This could be explained by several creations in 

 different places, with some hmited subsequent movement and 

 mixing, but it is equally well explained by a single origin of each 

 ancestor, followed by movement and evolution in different parts. 

 The regular occurrence on islands of animals similar to, but 

 specifically distinct from, those of the nearest mainland, coupled 

 with the fact that the greater the distance between island and 

 mainland the greater the difference in their faunas, strongly 

 suggests descent with modification — a common ancestor for each 

 related pair of species on island and mainland, followed by inde- 

 pendent evolution. The Galapagos Archipelago, situated on the 

 equator over five hundred miles from Ecuador, has a fauna which 

 is characteristically American in type, although most of the species 

 are found only in the islands. The Canary Islands, only a hundred 

 miles from Morocco, have a North African-European fauna, 

 with far fewer species of their own. In both groups of islands 

 there are some species, and more subspecies, confined to a single 

 island. Subspeciation is shown in Great Britain, for example in 

 the birds. Although it is doubtful if any of our species are speci- 

 fically distinct from those of the Continent of Europe, many are 

 of recognisably distinct subspecies, and the wren, a very sedentary 

 bird, although our mainland form is not distinguishable from that 

 of the Continent, has subspecies Troglodytes troglodytes hirtensis, 

 T. t. zetlandictis, and T. t. hehridensis, on St. Kilda, Shetland, and 

 the Outer Hebrides respectively. Other subspecies are found in 

 the Faroes, Iceland, Corsica, and Cyprus. 



The faunas of similar environments in different parts of the 

 world may be superficially similar, but they always show 

 differences corresponding to the differences between other animals 

 living in the same geographical region. This is well shown 

 wherever there are ecological islands, as, for example, the tops 

 of high mountains. The Andes have an alpine fauna, but it is 

 American ; that of the Sv/iss Alps is European. Caves show the 



