MEANS OF DISPERSAL. 5 1 



transport molluscs adhering to their coats, and many 

 other creatures which occasionally journey over land 

 have probably helped in the work ; but their influence, 

 that of mammalia at least, is doubtless chiefly local, 

 and must always be confined to a given tract of land. 

 Birds, it will be remembered^ commonly fly across arms 

 of the sea and are not unfrequently blown by gales over 

 wide stretches of ocean ; flying water-insects, also, 

 which may sometimes carry fry as well as ova, are 

 occasionally blown to great distances. 



It seems probable that young and small shells 

 may sometimes be transported along with the small 

 water-plants which occasionally adhere to birds. Mr. 

 Darwin, in removing a little duckweed from one 

 aquarium to another, unintentionally stocked the one 

 with fresh-water shells from the other, and he twice saw 

 duckweed adhering to the backs of ducks which had 

 suddenly emerged from a pond covered with these little 

 plants.^ It would be interesting to ascertain whether 

 small shells, and the fry of larger kinds, are often found 

 amongst such plants. A small tin cartridge-box full of 

 gibbous duckweed {Lemna gibbd) skimmed from the 

 surface of one of the streams by the Lea in Sep- 

 tember, contained a number of little aquatic animals, 

 shrimps, leeches, a beetle, etc., but no shells. A similar 

 quantity of lesser duckweed {Lemna minor)^ however, 

 carefully picked (in August) from the surface of a pond 

 near Louth, yielded no less than forty-two shells, 

 apparently alive : four of Sphcerium corneum and thirty- 

 ' "Origin," pp. 344-5. 

 E 2 



