COMPOSITION OF SEA-TVATEE. 141 



and to a whole organism are of no particularly prominent in- 

 terest to the question we are discussing ; on the other hand, it 

 will be necessary to investigate a number of special instances 

 of the effects of water more closely, in order to be able to 

 contrast those cases in which it gives rise merely to a selection 

 between different forms with others which prove that its in- 

 fluence is also capable of causing a true transformation. Few 

 as these latter cases are, for that very reason they have special 

 claim on our interest. 



ir. Effects of the chemical composition of the water. — In 

 proceeding to investigate these, it wiU be well in the first place 

 to direct our attention to two extreme cases — to the influence, 

 that is to say, of fresh and salt water. 



The salt savour of sea-water is occasioned, as is well known, 

 by the presence of a tolerably large proportion of sodic chloride, 

 generally known as common salt. When we speak of the 

 effects of sea- water on animal life, they are usually ascribed to . 

 this salt. Still, as many other substances are found in the sea 

 besides sodic chloride, siich as calcium salts, magnesium salts, 

 bromine, iodine, and other metals, carbonic acid, &c., it is to 

 be supposed that these are not wholly without importance in 

 the economy of animal life in the sea. But we do not know how 

 great their effects may be ; and as we are now perfectly accus- 

 tomed to attribute all the differences which have been observed 

 in the effects of sea-water, as distinguished from fresh water, 

 simply to its saltness, and to express the difference between 

 them in fractions per cent, of the amount of sodic chloride 

 held in solution, we will follow the usual custom in our enquiry, 

 without forgetting that at the same time numerous other 

 matters add their effects, though these are unknown, to those 

 of the pure sodic chloride. 



At the first glance we might feel inclined to explain the 

 fact, that a much gi'eater multiplicity of forms prevails in the 

 sea than in fresh water, by the supposition that the salt in the 

 ocean favours the production of variety in animals. It is 

 known that in recent times whole groups of animals are wholly 

 excluded from fresh waters, as the Eohinodermata, Sipunculidse, 

 polychsetous Annelida, Tunicata, Brachiopoda, and Cephalopoda, 



