MEANS OF DISPERSAL. 29 



But we must inquire as to the means by which the 

 creatures have been diffused. The problem, everyone is 

 aware^ is an old one, and has been answered or partially 

 answered by many writers ; Sir C. Lyell attended to it, 

 as also did Mr. Darwin, and many naturalists have 

 followed in the footsteps of these distinguished authors. 



In early life, collecting in the fresh waters of Brazil, 

 Mr. Darwin noted with surprise the similarity of the 

 fresh-water insects, shells, &c., and the dissimilarity of 

 the surrounding terrestrial forms of life, as compared 

 with those of this country ; and questions as to the 

 diffusion of the former seem to have been more or less 

 prominently before his mind during many years ; a 

 letter on means of dispersal of bivalves, one of his latest 

 published writings, appeared in " Nature ^' only thirteen 

 days before his death. In the " Origin " he expressed 

 the belief that the wide ranging power of fresh-water 

 productions could be explained, in most cases, " by their 

 having become fitted, in a manner highly useful to them, 

 for short and frequent migrations from pond to pond, 

 or from stream to stream, within their own countries," 

 and liability to wide dispersal, he remarked, '^ would 

 follow from this capacity as an almost necessary con- 

 sequence." ^ The way in which such migrations are 



rium comeiun, "Science Gossip," xx. (1884), 91 : C. T. Simpson, 

 on U7iio, &c., " Conchologists' Exchange," ii. (1887), 50 ; E. B. 

 Sanger, on an Australian fresh-water mussel, " American Nat.," 

 xvii. (1883), 1 184-5 ; ^J^d J. E. Gray, on Drezssena polymorpha, 

 "Annals of Philosophy," ix. (1825), 139. 

 1 " Origin," p. 343. 



