532 ISLAXD LIFE part ii 



direct result and outcome of a complex set of causes, which 

 may. be grouped as '• biological " and " physical." The 

 biological causes are mainly of two kinds — firstly, the 

 constant tendency of all organisms to increase in numbers 

 and to occupy a wider area, and their various powers of 

 dispersion and migration through which, when unchecked, 

 they are enabled to sj^read 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 lia\'e, 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 climate 

 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 o-eolooical climates and their causes, which have been 

 here investioated at some lenoth 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 stratified deposits, with a view to fix 

 within some limits their probable age ; and also to an 

 estimate of the probable rate of development of the organic 

 world ; and both these processes are shown to involve, so 

 far as we can judge, 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 jDhysical origin, in three groups or classes, each 



