GEOGRAPHIC ISOLATION 89 



birds. Some of these species occur on only one island, others on several, 

 sometimes with the formation of subspecies. 61 Their relations are 

 primarily American. The giant land tortoises, from which these islands 

 take their name, are represented by 14 species on the 9 largest 

 islands. 62 The genus of lizards Tropidurus is represented by separate 

 species or subspecies on each of 12 islands. 63 



The Azores present a contrast with the above-named archipelagoes, 

 as their fauna exhibits much less endemism. This group is composed 

 of 9 volcanic islands 1400 km. from the Portuguese coast. Thirty- 

 three of the 71 mollusks and 14 of the 74 species of beetles are 

 endemic; the single species of lizards is identical with a Madeiran 

 one; and of the 34 breeding birds, only one, Pyrrhyla pyrrhula murina, 

 is an endemic subspecies. 



It may be assumed with certainty that Madagascar, an island 

 large enough to be called a small continent, was once united with 

 Africa.* The peculiarity of its animal life makes it necessary to place 

 the date of this connection far back in geologic time, so that the 

 Madagascan fauna had a long and undisturbed period of development. 

 The dominant genus of earthworms is Kynotus, with 10 species, con- 

 fined to Madagascar. Among snails the genus Cyclostomus has a great 

 number of Madagascan species, with few elsewhere, while Ampelita 

 and Helicophanta are endemic. Forty-two of the 46 Madagascar 

 genera in the cetonid family of beetles are confined there. The am- 

 phibians and reptiles are represented almost entirely by peculiar 

 genera, some quite without existing allies in Africa. 



Two endemic genera of iguanid lizards (otherwise in America and 

 the Fiji Islands), and two of boid snakes, either identical with or very 

 closely allied to the American genera Constrictor and Boa, occur in 

 Madagascar. 



Of 28 genera of mammals, exclusive of bats, all but 3 are endemic. 

 Through the long-continued isolation, 3 distinct families have been 

 developed (or preserved). These are the primitive insectivores Cente- 

 tidae, with 7 genera and 18 species; the rodent family Nesomyidae, 

 with 7 genera and at least 12 species; and the subfamily of lemurs, 

 Lemurinae, with 12 genera and about 50 species. The Centetidae have 

 undergone adaptive radiation; some live like shrews or hedgehogs, 

 others, in the water like muskrats, and still others burrow like moles. 

 The dependence of the formation of species under isolation upon 

 the vagility of the groups of animals concerned is well illustrated by 



* Matthew regards Madagascar as an ancient oceanic island, never connected 

 with Africa. 



