104 
THE PRINCIPLES OF BIOLOGY. 
different areas in the same medium, and that over different 
media. The facts of distribution over different areas in the 
same medium are chiefly the following :—The fauna of a 
peculiar area is determined less by its fitness for certain types 
than by the possibility of its being populated from neighbour¬ 
ing areas. This is to say, we often find areas very much 
alike in climate, soil, and physical condition, but inhabited 
by quite different types, while on the other hand the same 
types are often found on areas differing very much in those 
respects. Now these facts at once contradict any theory of 
design. 
If we consider the latent tendency of all organisms to 
extend their sphere of existence we shall be able to under¬ 
stand these facts better. Supposing an organism inhabits a 
small area and has arrived at a complete harmony with its 
surroundings. Then it will surely begin to spread over the 
neighbouring areas, where, in the struggle for existence with 
already established forms, modifications must be produced, 
and the result is that there are two closely allied forms of 
the same type inhabiting adjacent areas. And that is, in fact, 
exactly what we find in Nature. This spreading of organisms 
over new areas is very well illustrated by the spreading of 
European animals and plants in New Zealand, where they 
overrun the country to the extinction, in frequent cases, 
of the native types. 
In the last case we assumed that there was no barrier to 
the spread of the organism. If there are such, the present 
state of the fauna and flora will entirely depend on the length 
of time these barriers have existed. If they had been in 
existence for a long time, we should expect that although 
before their appearance the faunas were closely allied, they 
will not be so now, as ample time has elapsed for their 
modification. The modifications will be the more important 
the longer the time that has passed since the last connection. 
We find yery striking illustrations of these facts in the study 
of the fauna and flora of islands; such as the Azores have 
only a small number of different types, and all are allied to 
those of the nearest continents. In fact, the organic forms 
are only derived from chance arrivals from the shores of 
Africa and Europe. They are nearly all such forms as have 
special means of spreading or migrating, or, like birds and 
insects, are winged animals, which may be carried away by 
the wind. Some types are much modified, and they must be 
the descendants of continental forms which arrived there at 
a very remote time. Others are very little modified, and do 
not trace their pedigree quite so far back. Others, again, are 
