332 THE NATURALIST IX NICARAGUA. [Ch. XVIII. 



remote islands,* The close affinities of fresh-water 

 animals and plants have been noticed by many natu- 

 ralists. Darwin saw with surprise, in Brazil, the simi- 

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

 dissimilarity of the surrounding terrestrial beings com- 

 pared with those of Britain, f Dr. D. Sharp informs 

 me that water-beetles undoubtedly present the same 

 types all over the world. He believes there is no 

 family of Coleoptera in which tropical or extra- tropical 

 species so closely resemble one another as in the 

 DytiscidcB. Cybister is found in Europe, Asia, Africa, 

 Australia, New Zealand, Brazil, and North America ; 

 and the species have a very wide range. Dr. Sharp 

 remarks that this wide distribution and great simi- 

 larity of the DytiscidcB is of special interest when we 

 recollect that they are nothing but CardbidcB fitted for 

 swimming, and yet that the Carabidre are one of the 

 groups in which the tropical members differ widely from 

 the temperate ones. 



For following up this branch of inquiry the study of 

 the distribution of the mollusca offers special advantages. 

 There are numerous marine, fresh-water, and terrestrial 

 species and genera. They are slow moving ; they have 

 not the means of transporting themselves great distances, 

 like insects, for example, that may easily and often pass 

 over arms of the sea, or fly from one country to another. 

 Their shells are the commonest of fossils ; and in islands 

 such as Madeira and St. Helena, where we have abundant 

 remains of extinct land shells, there are few, if any, of 

 extinct animals of other classes or of plants. 



Taking the shells of Europe, we find a remarkable dif- 



Darwin, " Origin of Species," p. 417. f Ibid. p. 414. 



