356 
DARWINISM 
CHAP. 
competitors among which they have been developed. Such 
birds as these may pass again and again to a new country, but 
are never able to establish themselves in it; and it is this 
organic barrier, as it is termed, rather than any physical 
barrier, which, in many cases, determines the presence of a 
species in one area and its absence from another. We must 
always remember, therefore, that, although the presence of a 
species in a remote oceanic island clearly proves that its 
ancestors must at one time have found their way there, the 
absence of a species does not prove the contrary, since it also 
may have reached the island, but have been unable to main¬ 
tain itself, owing to the inorganic or organic conditions not 
being suitable to it. This general principle applies to all 
classes of organisms, and there are many striking illustrations 
of it. In the Azores there are eighteen species of land-birds 
which are permanent residents, but there are also several 
others which reach the islands almost every year after great 
storms, but have never been able to establish themselves. In 
Bermuda the facts are still more striking, since there are only 
ten species of resident birds, while no less than twenty other 
species of land-birds and more than a hundred species of 
waders and aquatics are frequent visitors, often in great 
numbers, but are never able to establish themselves. On 
the same principle we account for the fact that, of the many 
continental insects and birds that have been let loose, or 
have escaped from confinement, in this country, hardly 
one has been able to maintain itself, and the same pheno¬ 
menon is still more striking in the case of plants. Of the 
thousands of hardy plants which grow easily in our gardens, 
very few have ever run wild, and when the experiment 
is purposely tried it invariably fails. Thus A. de Candolle 
informs us that several botanists of Paris, Geneva, and 
especially of Montpellier, have sown the seeds of many 
hundreds of species of exotic hardy plants, in what appeared 
to be the most favourable situations, but that in hardly a 
single case has any one of them become naturalised. 1 Still 
more, then, in plants than in animals the absence of a species 
does not prove that it has never reached the locality, but 
merely that it has not been able to maintain itself in com- 
1 Geographie Botanique, p. 798. 
