500 
ISLAND LIFE. 
[part II. 
kinds — firstfy, 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 
