MEANS OF DISPERSAL. 37 



adhering to every available object ; a detached bracken- 

 frond lying in the water could only be described as 

 literally covered ; I tried to count the number of masses 

 upon it, but soon gave up the task ; there were hun- 

 dreds, and possibly each contained from fifty to sixty 

 eggs.^ An object of this kind, one can readily imagine, 

 if near a stream which occasionally overflowed, might 

 easily be carried away with the flood-water, and, if it 

 happened to reach a pond or other water where the 

 conditions were favourable, the introduced species could 

 hardly fail to establish itself in the new home. 



Mr. C. T. Simpson has even expressed the opinion 

 that in some parts of the world trees, &c., drifted down 

 by rivers and floods, may occasionally float fresh-water 

 molluscs in safety over the sea. In his own words : 



" Great numbers of trees are washed out and 

 lodged along the rivers and torrents of tropical 

 countries ; and in the crevices of the bark of these 

 trees many species of fresh-water shells find a home. 

 Others live among the roots of living trees which 

 are washed by the water of streams. In Florida I 

 have collected handfuls of Unio fiiscatus and other 

 species among the matted roots of trees just under the 

 surface of the water. Such trees, washed out and 

 carried down stream, would take some of their molluscan 

 inhabitants with them. Others, carried in floods, with 

 broken jagged limbs and loosened bark, would plow up 

 quantities of mud and shells from the bottom and carry 



* See " Rimmer," p. 62. 



