THE PRINCIPLES OF BIOLOGY. 
105 
identical with continental species, and these are found 
among the regular arrivals from the continent during storms, 
and as the new arrivals mix with the natives they thus 
prevent modifications. 
But if we take the fauna and flora of St. Helena, for 
example, we find that there are only very few species identical 
with continental ones ; a greater part are allied forms, and 
the greatest proportion are entirely peculiar forms found 
nowhere else. The reason is that St. Helena is farther from 
any continent, and has never been in connection with one ; 
while the Azores are at a smaller distance, and although 
perhaps they have never been in direct communication with 
the continent, it is possible that the distance at a remote 
period was smaller still, and broken by intervening islands. 
From the principles of the theory of evolution we should 
also expect that those areas which have been isolated for the 
longest time possess the lowest types ; and that is actually 
the case, as is shown by the peculiar fauna of Australia, 
almost the only country where the marsupials, the lowest 
type of mammals, are preserved. 
There is also no difficulty in explaining the distribution 
of organisms over different media. As we find the lowest 
organisms in water, we must suppose that the beginning of 
life occurred in that medium, and we must explain how it 
was possible for organic forms to spread from water to land. 
The first beginning of a change in that direction might have 
been caused by agencies like the tide. Along the shores of 
the sea there is a large number of animals which regularly 
spend part of their life in water and part in air. There are 
fishes which fly through the air or climb trees ; birds which 
dive. There are animals living in mud pools which during 
the dry season have to live without water. There are the 
amphibia, most of which can live in either medium, but 
some only inhabit one, although they are able to live in the 
other. We have thus a perfect gradation from animals 
entirely confined to water through such as can live in either 
to those which can only live in air; and that is just what 
we should expect, wliat is a necessity from the principles of 
evolution. 
When we consider the distribution in time we find that 
the evidence is very scanty, but such as it is, it is entirely on 
the side of evolution. The fact that the difference in fossil 
remains is proportional to the difference in the time of their 
existence; the fact that species, genera, and entire types 
are extinct; the fact that the length of existence of a species 
does not necessarily imply a large amount of modification ; 
all those facts are in harmony with the evolution theory. 
