82 THE DISPERSAL OF SHELLS. 



they must often chance to be set down in unsuitable 

 spots ; it is true, as Mr. Darwin has remarked, that a 

 bird, such as a duck or heron, "if blown across the sea 

 to an oceanic island or to any other distant point, would 

 be sure to alight on a pool or rivulet,'' ^ but probably a 

 bivalve would generally remain attached for some time 

 after the bird had landed even if kept much under 

 water. Sooner or later, however, the valves would have 

 to be opened, and this is more likely to happen in 

 water than on land. It may be seriously objected, I 

 admit, that the transplantation of a shell full of fry or 

 ova must be a comparatively rare occurrence, and it 

 is even doubtful, perhaps, whether a single 'individual in 

 this condition would have much chance of establishing 

 a colony in a new home. Occurrences of the kind 

 dealt with in this chapter are perhaps, after all, chiefly 

 significant when viewed in connection with anomalies 

 in local distribution, and it can scarcely be doubted but 

 that they go far towards explaining the almost mysteri- 

 ous presence of bivalves in isolated ponds, between 

 which and other waters a more or less constant com- 

 munication is kept up by animals of many kinds, 

 especially by flying water-insects and aquatic birds. 



Many creatures, other than those above referred to, 

 are doubtless occasionally entrapped by bivalves. 

 Three cases of which I have heard are perhaps worth 

 giving, but they are not of much importance for us. 



i •' Origin," p. 345. 



