298. THE INFLUENCE OF INANIMATE SURROUNDINGS 



unlike their congeners living on land, display an extraordinary 

 range of identical species. Eggs or young individuals, so he 

 argues, might make very long journeys adhering to the webbed 

 feet of a migratory duck ; and as such journeys must be frequently 

 repeated, according to the constant direction of the migrations 

 or circuits of the bird, numerous specimens of the same species 

 of water-mollusc must traverse the same route. The accumu- 

 lation of individuals of the parent species in the same colony 

 thus caused would prevent the formation of a new species, since 

 the selective influence of the struggle for existence in the new 

 conditions of life would be constantly counteracted by the re- 

 peated immigration of individuals of the parent form. 



This explanation is satisfactory, and in many cases it cer- 

 tainly seems to be the right one, as, for instance, in the case 

 of the distribution of the European fresh- water mollusca. Still 

 very considerable difficulties stand in the way of its exten- 

 sive application. It must, in the first place, be observed 

 that the great similarity of the fresh-water mollusca throughout 

 the globe, assumed by Darwin, does not exist to such an extent 

 as might be supposed from what he says. This contradiction 

 on my part requires some explicit verification. The genus 

 Unio, for instance, is distributed almost everywhere on the 

 face of the globe ; it is absent only from a few tropical 

 countries, as the Moluccas, the islands of the Pacific, probably 

 New Guinea, and others ; but the species of Unio are extra- 

 ordinarily various; in North America almost every little 

 stream has its own peculiar form, and the European, Asiatic, 

 and Australian species are widely dissimilar. These differences 

 may be even greater and more striking than we now suppose, 

 for we have hardly exact knowledge enough of the organic 

 characters of more than a few dozen forms to venture to 

 pronounce a decisive opinion, while hundreds of species are as 

 yet known to us only by their shells. The other genus of 

 fresh-water mussels, Anodonta, has an even wider range, for it 

 occurs even in those islands where Unio is wanting. But for 

 this genus also these remarks hold good. Among the Univalves 

 the genera Melania and Paludina (fig. 77) have a very wide 

 distribution, and exhibit a considerable resemblance in the 



