FORMER LAND CONNECTIONS. 1211 



The entire duration covered by fossil forms has for con- 

 venience been divided into epochs, each of which covers a period, 

 marked by the development of some important order of animal or 

 plant life. For example, we have the Devonian characterised by 

 the rise of the fishes, the Carboniferous of the Amphibia, the 

 Permian of the Reptilia, the Triassic of the Dinosauria, the Jurassic- 

 of the ammonites and birds, the Cretaceous of the angiospermous 

 plants, and the Tertiary of the mammals. 



The time covered by each of these epochs is not determinable, 

 but must run into millions of years. Although there was, a9 

 already stated, at any one time a similarity, often wonderful, in 

 the life of the various parts of the globe, detailed study has shown 

 that then, just as now, there existed differences of flora or fauna in 

 particular regions, largely owing to the effects of environment or 

 to migration. 



It is well known that the marine faunas show less variation 

 in this respect than land ones, the departure from the normal being 

 most where the waters are confined, as for example in the Black 

 Sea. Generally the migration of forms and their dispersal is not 

 so much restricted in the ocean. 



On the land, however, the spreading of animals and still more • 

 of plants is greatly hampered by barriers, physical, climatic or 

 biological, with the result that the life in adjoining areas may come 

 to differ very considerably. 



Former Land Connections. 



Particularly for the reason just mentioned must one avoid' 

 concluding that when the geological evidence points to the prob- 

 able existence of a particular land-bridge in the past, some allied 

 or even identical genera or species, extinct or living, ought to be 

 found along its entire length. 



Again to infer, because certain forms were able to migrate* 

 from one region to another, that a simultaneous interchange should 

 have taken place between the two regions, is contrary to observa- 

 tion. 



Where the two lands are close together the geological data may 

 be sufficient to establish their former connection, but, where • 

 thousands of miles of ocean are separating them, the evidence 

 demanded is mainly biological and the arguments have obviously 

 to be weighty before geographical changes of such vast magnitude • 

 and revolutionary nature can be admitted. 



It may happen, upon the breaking up of a continental mass 

 into a series of islands, that of the forms thus isolated extinction 

 overtakes certain of them through various natural causes, and at 

 length they may come to be represented by but a few allied species 

 in lands now parted bv the ocean. For certain reasons it is 

 commonly the more primitive forms that have thus become segre- 

 gated. 



It must frankly be admitted that land-connections between the 

 continents have frequently been postulated upon very slender 

 grounds and the strictures by Darwin and by Matthews in this-- 



