280 INTRODUCTION TO EVOLUTION 



must be drawn as to the beginnings of most of the "experiments." Never- 

 theless, the inferences can in many cases be made with considerable con- 

 fidence. Thus oceanic islands have always held particular fascination for 

 students of geographic distribution. 



Continental Islands 



Islands in the oceans of the world are classified into two categories: 

 continental islands and oceanic islands. Continental islands are those which 

 were at some time in the past connected to a continent; oceanic islands 

 are those which have never been connected to a continent. For the most 

 part, the former are located on the continental shelves of the continents 

 nearest them; they are usually connected to the neighboring continent by 

 shallow ocean, 100 fathoms (600 feet) or less in depth. (Note that in 

 Figs. 12.4 and 12.5, pp. 269 and 271, certain regions of the ocean border- 

 ing continents and lying between them and neighboring islands have been 

 left unshaded. These are the regions which are 100 fathoms or less in 

 depth.) The maps indicate, therefore, that if the land were raised, or the 

 ocean lowered, by 100 fathoms many islands would be connected to con- 

 tinents near them. For example, the British Isles would be connected to 

 Europe (Fig. 12.5); Borneo, Sumatra, Java, and neighboring islands 

 would be connected to southeastern Asia; New Guinea would be connected 

 to Australia; Ceylon would be connected to India; Japan would be con- 

 nected to Asia; and so on. All these, then, are continental islands, since 

 there is no doubt that the ocean level has in times past fluctuated at least 

 as much as the amount indicated. 



In plant and animal life, continental islands are characterized by resem- 

 blance to the continent to which they were formerly joined. Naturally the 

 resemblance is greatest if the connection to the continent was recent in 

 geologic time, less if the connection was more remote in time, permitting 

 the appearance meantime of new forms on the continent, and perhaps also 

 on the island. Of particular interest in comparision to the fauna of oceanic 

 islands is the fact that co ntinenta l islan ds possess amphibians_and also the 

 large mammals ch aracter istic of the neighbormg continent. Thus, the mam- 

 Inais^f the British Isles are in the main identical with those of Europe; 

 and Borneo, Java, and Sumatra have tapirs, rhinoceroses, deer, wild dogs, 

 members of the cat family, and other mammals found on the neighboring 

 Asiatic mainland. It is most unlikely that such a mammalian fauna would 

 have reached these islands except over dry land. 



