520 LAND ANIMALS 



from the nearest continent, have about 500 species of land snails, all 

 of which are endemic, 1288 species of beetles, with 1107 endemic, and 

 48 species of land birds, exclusive of birds of prey, all of which are 

 endemic. 11 The relation between speciation and insular isolation is well 

 shown by the 201 species of island-inhabiting land planarians known 

 up to 1899, of which no less than 186 are confined each to a single 

 island. 12 The island-inhabiting cassowaries are split into 20 subspecies 

 by Rothschild, while the continental ratite birds are much less diversi- 

 fied — the ostriches in 4, the rheas in 4, and the emus in only 3 sub- 

 species. The development of subspecies by insular isolation is still 

 further emphasized by comparison of an archipelago with a near-by 

 mainland area. The Philippines, for example, have 1079 species of land 

 snails (without enumerating subspecies), while there are only 618 

 species in Indo-China and Siam. 13 The West Indies have 64 amphibians 

 and 261 reptiles, as compared with 26 amphibians and 83 reptiles on 

 Celebes, which has about an equal area. 



Isolation on islands affords effective protection against the entrance 

 of competitors, and with the relatively smaller number of forms on a 

 single island, the struggle for existence is less severe. Forms which have 

 succumbed to the struggle with more advanced types in mainland 

 areas may accordingly survive on islands. The small operculate snail, 

 Craspedosoma azoricum, persists in the Azores, while its only relatives 

 are known from Tertiary deposits in Europe. 14 The helicid genus 

 Jamdus, of the continental Oligocene and Miocene, persists in 

 Madeira. 15 Primitive forms of the most diverse groups of animals have 

 been preserved in New Zealand. The Tasmanian wolf (Thylacinus) , 

 and the Tasmanian devil, are only recently extinct in Australia, where 

 they appear to have been displaced by the dingo, which did not reach 

 Tasmania. Madagascar affords a further example; lemurs, and civets, 

 and primitive insectivores, in the absence of true monkeys and true 

 cats, together with the paucity of ungulates, mark its fauna off from 

 Africa, from which it has been separated probably since mid-Tertiary. 

 Modern groups tend to be represented by their more primitive 

 members. 



The absence of native mammals, bats excepted, or their scarcity 

 on islands, is especially favorable for bird life. The rather helpless 

 pigeons increase in numbers in the Malay Archipelago to the eastward 

 as the mammals decrease, and are strikingly developed in Polynesia, 

 where no native mammals exist; the distinctive family Didunculidae 

 is confined to two islands of Samoa. For the same reason, islands in 

 the tropics, as well as in the arctic and antarctic, afford undisturbed 



