Islands, because of their relatively limited area and their clearly defined boundaries, are excellent fields for the study of floral, faunal, and ethnic distribution.  Small area and isolation cause in them poverty of animal and plant forms and fewer species than are found in an equal continental area.  This is the curse of restricted space which we have met before.  The large island group of New Zealand, with its highly diversified relief and long zonal stretch, has only a moderate list of flowering plants, in comparison with the numerous species that adorn equal areas in South Africa and southwestern Australia.[812] Ascension possessed originally less than six flowering plants.  The four islands of the Greater Antilles form together a considerable area and have all possible advantages of climate and soil; but there are probably no continental areas equally big and equally favored by nature which are so poor in all the more highly organized groups of animals.[813] Islands tend to lop off the best branches.  Darwin found not a single indubitable case of terrestrial mammals native to islands situated more than three hundred miles from the mainland.[814] The impoverishment extends therefore to quality as well as quantity, to man as well as to brute.  In the island continent of Australia, the native mammalia, excepting some bats, a few rodents, and a wild dog, all belong to the primitive marsupial sub-class; its human life, at the time of the discovery, was restricted to one retarded negroid race, showing in every part of the island a monotonous, early Stone Age development.  The sparsely scattered oceanic islands of the Atlantic, owing to excessive isolation, were all, except the near-lying Canaries, uninhabited at the time of their discovery; and the Canary Islanders showed great retardation as compared with their parent stock of northern Africa. [See map page 105.]

[Sidenote:  Endemic forms.]

Despite this general poverty of species, island life is distinguished by a great proportion of peculiar or endemic forms, and a tendency toward divergence, which is the effect of isolation and which becomes marked in proportion to the duration and effectiveness of isolation.  Isolation, by reducing or preventing the intercrossing which holds the individual true to the normal type of the species, tends to produce divergences.[815] Hence island life is more or less differentiated from that of the nearest mainland, according to the degree of isolation.  Continental islands, lying near the coast, possess generally a flora and fauna to a large extent identical with that of the mainland, and show few endemic species and genera; whereas remote oceanic islands, which isolation has claimed for its own, are marked by intense specialization and a high percentage of species and even genera found nowhere else.[816] Even a narrow belt of dividing sea suffices to loosen the bonds of kinship.  Recent as are the British Isles and near the Continent, they show some biological diversity from the mainland and from each other.[817]