TRANSPLANTATION OF BIVALVES. 67 



in the evening or at night somewhat frequently, and it 

 is perhaps worth mentioning that at the Linnean 

 Society in 1852, John Curtis, the distinguished en- 

 tomologist, expressed the opinion that the larger aquatic 

 insects — especially the Dytiscidae — might without doubt 

 be the means of conveying fish-spawn from one piece of 

 water to another,^ and Mr. Wallace, in like manner, 

 discussing the means of dispersal of fishes, observes that 

 water-beetles " flying from one pond to another " may 

 occasionally carry eggs.'-^ No doubt, as Mr. Darwin has 

 remarked, the creatures generally alight on any pool of 

 water they may see, for there is no reason to suppose, 

 as far as I know, that they are in the habit of ^' homing" 

 or returning to the same pool from time to time. But 

 another difficulty has to be met. At the Entomological 

 Society in 1888, when I showed the West Barkwith 

 specimen of D. marginalis with the attached shell of 

 Sphcerium ccrneum, and ventured to suggest that 

 occurrences of the kind indicated were probably of 

 significance when viewed in connection with the dis- 

 tribution of bivalves. Dr. David Sharp, the president, 

 objected that the weight of the mollusc would in all 

 probability prevent or greatly impede the beetle's flight, 

 and this objection, no doubt, will have suggested itself to 

 many. Mr. Darwin, however, has expressed the opinion 

 that six grains (the weight of an individual of S. 

 corneum of average size) would not prevent so powerful 



' See J. Hogg, "Trans. Tyneside Nat. Field Club," iii. (1854- 



8). 75- 



2 Wallace, "Geographical Distribution," i. (1876), 29. 



F 2 



