DISPERSAL BY MAN. 195 



after about a year.' Mr. Baillie tells me that he put 

 down a few specimens of Avialia marginata in his 

 garden — in Sutherlandshire — about nine years ago, but 

 nothing has been seen of them since. 



We have now to inquire as to the means by which 

 molluscs may have been unintentionally dispersed by 

 man's agency. 



Fresh-water Shells. 



Aquatic shells and their ova — no doubt less frequently 

 dispersed by man than terrestrial kinds — seem likely to 

 have been occasionally carried with cargoes and con- 

 signments of various sorts. One species now flourishing 

 in this country, as we shall see, is believed to have been 

 imported attached to timber in the holds of ships, and 

 another is supposed to have come from America in 

 cotton bales. Our Liuincsa stagnalis, it is suggested, was 

 introduced into Tasmania, I presume unintentionally, 

 with fish-ova." Many kinds, both ova and adults, must 

 certainly have been carried from place to place, even to 

 great distances and from one country to another, along 

 with consignments of plants, but I am not aware that 

 the creatures are ever actually noticed by those who 

 unpack such consignments ; several facts, however, 

 clearly indicating transportal in this way might be given. 

 Thus, for instance, Mr. W. W. Westgate of Houston, 

 Texas, once found his water-lily tubs, in which plants 

 from Fayette County, Texas, and from Florida were 



* W. Jeffery, "Journ. of Conch.,"iii. (1882), p. 313. 

 ^ W. F. Pettard, "Journ. of Conch.," ii. (J879), p. 81, 



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