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. \Ye 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 %asitors, 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 natui'alised.^ 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- 

 ^ Geographic Botanique, p. 798. 



