300 THE INFLUENCE OF IKANIJIATE SURROUNDINGS. 



but the species of Lymnceri, Plano7-his, Physa, and Succinea, 

 which also live in fresh water (see fig. 78), have no such pro- 

 tection, and the month of the shell is remarkably large. 

 Research has shown that, as a fact, operculated snails resist 

 injurioixs influences far more successfully than those without an 

 operculum. According to this the species of the inoperculated 

 fresh-water univalves ought to exhibit a much sharper differen- 

 tiation into separate species according to their habitat than the 



f- 





FjGi. 78. — Vai'ious fresli-water suails. a, Lijuuura ; b, Succinea ; c, Pfiysa ; d, Planorbis. 



operpulated forms ; but the fact is precisely the contrary. 

 Analogous examples of other fresh-water animals could easily 

 be adduced. Thus, for instance, it is impossible to explain the 

 existence of Teinnocphala clvUfnuis (see fig. 76) in Chili, the 

 Philippines, and Java, by supposing it to have been carried 

 thither by birds, for it deposits its eggs in its host ; and these 

 are creatures much too large to have been carried alive by 

 birds across the ocean. 



It would certainly be, I will not say a grateful, but a very 



