500 



ISLAND LIFE. 



[part II. 



kinds — firstty, the constant tendency of all organisms to in- 

 crease in numbers and to occupy a wider area, and their 

 various powers of dispersion and migration through which, when 

 unchecked, they are enabled to spread widely over the globe ; 

 and, secondly, those laws of evolution and extinction which 

 determine the manner in which groups of organisms arise and 

 grow, reach their maximum, and then dwindle away, often 

 breaking up into separate portions which long survive in very 

 remote regions. The physical causes are also mainly of two 

 kinds. We have, first, the geographical changes which at one 

 time isolate a whole fauna and flora, at another time lead to 

 their dispersal and intermixture with adjacent faunas and floras 

 — and it was here important to ascertain and define the exact 

 nature and extent of these changes, and to determine the 

 question of the general stability or instability of continents 

 and oceans ; in the second place, it was necessary to determine 

 the exact nature, extent, and frequency of the changes of cli- 

 mate which have occurred in various parts of the earth, — 

 because such changes are among the most powerful agents in 

 causing the dispersal and extinction of plants and animals. 

 Hence the importance attached to the question of geological 

 climates and their causes, which have been here investigated 

 at some length with the aid of the most recent researches of 

 geologists, physicists, and explorers. These various inquiries 

 led on to an investigation of the mode of formation of strati- 

 fied deposits, with a view to fix within some limits their pro- 

 bable age ; and also to an estimate of the probable rate of 

 development of the organic world; and both these processes 

 are shown to involve, in all probability, periods of time less 

 vast than have generally been thought necessary. 



The numerous facts and theories established in the First 

 Part of the work are then applied to explain the phenomena 

 presented by the floras and faunas of the chief islands of the 

 globe, which are classified, in accordance with their physical 

 origin, in three group or classes, each of which are shown to 

 exhibit certain well-marked biological features. 



Having thus shown that the work is a connected whole, 

 founded on the principle of tracing out the more recondite 



