30 THE DISPERSAL OF SHELLS. 



probably effected was discussed in some detail, and 

 diversified and curious means of dispersal were shown 

 to be in operation ; as regards molluscs, facts were given 

 suggesting transportal, possibly over the sea, by aquatic 

 birds and insects;^ and subsequently, in 1882, when 

 publishing several additional facts indicating dispersal 

 by animal agencies, Darwin demurred to the belief, 

 which had been somewhat doubtfully expressed by Mr. 

 Jeffreys, that the diffusion of fresh-water shells " had a 

 different and very remote origin, and that it took place 

 before the present distribution of land and water." " 



Much diffusion, of course, as Darwin admitted, must 

 be attributed to changes in the level of the land, " within 

 the recent period, causing rivers to flow into each other." ' 

 Streams belonging to distinct systems, Mr. Wallace has 

 remarked, often approach and might be made to change 

 their course from one to the other basin by very slight 

 alterations of level. During the glacial epoch, ice, 

 blocking up valleys, compelled streams to flow over 

 watersheds to find an outlet'' 



The carrying powers of floods, as well as of rivers in 

 their ordinary action, are doubtless very great, and must 

 have largely influenced distribution. Professor Ralph 

 Tate, indeed, long ago expressed the opinion that the 



' " Origin," pp. 344.5. 



"^ Darwin, "Nature," xviii. (1878), 120-1; xxv. (1882), 529-30; 

 Jeffreys, " British Conchology," i. (1862), Ixxx. 



' " Origin," p. 344. 



* Wallace, " Island Life," (1880), pp. 74 and 324; ed. 2, pp. 76 

 and 344. 



