826 



it is an introduced species in Norway as well as in England), the 

 cockroach {Blatta orientalis), probably brought over in ships from the 

 Levant, and the fresh-water mussel {Dreissena polymorpha). These 

 species are mentioned towards the end of Mr. Marshall's very inte- 

 resting letters on Anacharis (Phytol. iv. 714) ; but the diffusion of 

 Dreissena polyraorpha in our canals (though, from the harmless and 

 obscure habits of the animal, it has not attracted general observa- 

 tion) bears so many points of resemblance to the rapid spread of Ana- 

 charis, that I may be pardoned for extracting the following passage 

 from the ' British Mollusca ' of Prof. Forbes and Mr. Hanley : — 



" These mussels live gregarious, often attached in great numbers to 

 each other, in fresh and brackish waters. Originally, apparently, 

 inhabitants of the rivers around the Black Sea, they have gradually 

 extended their range all over Europe ; capable of enduring salt-water 

 for a time, they have, probably, been carried across seas on the bot- 

 toms of ships, and in this manner have reached England and become 

 so common in our canals, as to be much more abundant than many 

 of our indigenous mollusks. Its history as a British species, dates 

 from 1824, when Mr. J. de Carle Sowerby exhibited it to the Lin- 

 nean Society, stating that it was found ' in abundance, attached to 

 shells and timber, in the Commercial Docks, by James Bryant, Esq., 

 who used the animal as bait for perch.' Mr. Stark found it in the 

 Union Canal, near Edinburgh, in 1834, and the Rev. M. J. Berkeley 

 observed it in the Nen, in 1836. In the latter case, the discoverer 

 believed it had been introduced from Wisbeach, on timber, in 1828. 

 Thus, it would appear to have found its way into Britain, at several 

 points, and is now common in the canals of the Midland and North- 

 ern counties." — Brit. Moll. ii. 167. 



The only striking instance of the rapid and unwelcome spread of 

 introduced plants which I have been able to find, at all resembling 

 the case of our Anacharis, occurred to me, unexpectedly, yesterday, 

 whilst cutting my way (paper-knife in hand) through the pleasant 

 pages of the recently-published second edition of Schleiden's ' The 

 Plant.' At page 350 of that work we read : — 



" The Pampas of Buenos Ayres have a character similar to that of 

 the North American prairies, only man by his influence upon Nature 

 has here and there impressed a peculiar stamp. The thistle and arti- 

 choke coming with the Europeans have quickly made themselves 

 masters of the free soil, and with incredible rapidity overspread dis- 

 tricts of many square miles with their spiny vegetation, which has 

 developed in a luxuriance unknown in Europe. These thistle-wastes 



