348 AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



existed from very remote times within the present continental 

 areas. It is perhaps best to accept this theory only for the 

 deeper parts of the great oceans. An accurate knowledge of the 

 contours of the ocean floor is important in this connection, and 

 serves as an important check upon speculations regarding former 

 land-unions. On this basis islands have been divided into oceanic 

 and continental, which are believed respectively to have been 

 always isolated, and to have been connected with an adjoining 

 continent. 



Oceanic Islands are : (a) generally remote from continents ; (&) 

 separated from them by very deep (usually over 1,000 fathoms) 

 water; (c) of volcanic or coral nature; (d) inhabited by forms 

 which possess powers of migration capable of carrying them, 

 actively or passively, over more or less broad ocean tracks ; (e) 

 characterised by numerous peculiar species. Exs. The Azores, 

 St. Helena, Ascension, coral islands of Pacific. 



Continental Islands are: (a) comparatively near a continent; 

 (b) separated from it by comparatively shallow (under 1,000 

 fathoms) water ; (c) of similar geological structure, and not 

 entirely volcanic or coral ; (d) inhabited by similar organisms, 

 irrespective of powers of migration. Such islands are : (1) 

 Ancient continental, separated from the nearest continent by fairly 

 deep (over 100 fathoms) water, and presenting only a general 

 resemblance in the fauna and flora ; many peculiar species. Exs. 

 Madagascar, Celebes. (2) Recent continental, separated from the 

 adjacent continent by shallow (not more than 100 fathoms) 

 water, and with closely similar fauna and flora ; very few peculiar 

 species. Exs. British Islands, Japan. Both (1) and (2) have 

 presumably been united with the adjoining continents, the latter 

 at a recent date, geologically speaking. 



The surface of the globe has also undergone numerous muta- 

 tions as regards climate. The temperate parts of N. America 

 and Europe, for example, were at a geologically recent period 

 passing through a glacial epoch (the " great ice age "), as proved 

 by ice-worn and scratched rock-surfaces and rocks, boulder-clay, 

 &c. On the other hand, fossil plants evidencing subtropical 

 conditions have been found in the Arctic regions. Many theories 

 have been advanced to account for secular changes of climate. 

 The most satisfactory is one by Wallace which attributes them to 

 geographical revolutions (previously suggested by Lyell), influ- 



