Now, when the division of the area leaves one portion of the species in an island, a similar modification of the species, either in the island or in the continent, occurs, resulting in closely-allied but distinct forms; and such forms are, as we have seen, highly characteristic of island-faunas. But islands also favour the occasional preservation of the unchanged species--a phenomenon which very rarely occurs in continents. This is probably due to the absence of competition in islands, so that the parent species there maintains itself unchanged, while the continental portion, by the force of that competition, is driven back to some remote mountain area, where it also obtains a comparative freedom from competition. Thus may be explained the curious fact, that the species common to Formosa and India are generally confined to limited areas in the Himalayas, or in other cases are found only in remote islands, as Japan or Hainan. The distribution and affinities of the animals of continental islands thus throws much light on that obscure subject--the decay and extinction of species; while the numerous and delicate gradations in the modification of the continental species, from perfect identity, through slight varieties, local forms, and insular races, to well-defined species and even distinct genera, afford an overwhelming mass of evidence in favour of the theory of "descent with modification." We shall now pass on to another class of islands, which, though originally forming parts of continents, were separated from them at very remote epochs. This antiquity is clearly manifested in their existing faunas, which present many peculiarities, and offer some most curious problems to the student of distribution. * * * * * {411} CHAPTER XIX ANCIENT CONTINENTAL ISLANDS: THE MADAGASCAR GROUP Remarks on Ancient Continental Islands--Physical Features of Madagascar--Biological Features of Madagascar--Mammalia--Reptiles--Relation of Madagascar to Africa--Early History of Africa and Madagascar--Anomalies of Distribution and How to Explain Them--The Birds of Madagascar as Indicating a Supposed Lemurian Continent--Submerged Islands between Madagascar and India--Concluding Remarks on "Lemuria"--The Mascarene Islands--The Comoro Islands--The Seychelles Archipelago--Birds of the Seychelles--Reptiles and Amphibia--Freshwater Fishes--Land Shells--Mauritius, Bourbon, and Rodriguez--Birds--Extinct Birds and their Probable Origin--Reptiles--Flora of Madagascar and the Mascarene Islands--Curious Relations of Mascarene Plants--Endemic Genera of Mauritius and Seychelles--Fragmentary Character of the Mascarene Flora--Flora of Madagascar Allied to that of South Africa--Preponderance of Ferns in the Mascarene Flora--Concluding Remarks on the Madagascar Group.