Cii INTRODUCTION. [CH. 



nature of their habitat in this respect. Mr. Spence Bate, 

 who has so diligently and successfully studied our native 

 shrimps, informs me that Gammarus locusta, which only 

 inhabits the sea, is scarcely distinguishable from G. flu- 

 viatilis, which would be instantly killed by being put 

 into salt water. Professor Lilljeborg has discovered in 

 some of the inland freshwater lakes of Denmark several 

 Arctic species of marine Crustacea, which appear to have 

 survived the Glacial epoch, and to have adopted from 

 necessity a new habitat, in consequence of the gradual 

 elevation of the land. And the result of the researches 

 made by Dr. E. von Martens on the occurrence of 

 marine animal forms in fresh water, which was published 

 in ' Wiegmann's Archiv ' for 1857, shows that 10 out of 

 44 divisions or groups of Crustacea, and 6 out of 52 

 divisions of Mollusca, are common to the sea and fresh 

 water. Fish have no less than 23 out of 55 divisions 

 similarly circumstanced as to habitat ; but some of these 

 are well known to migrate annually from the sea to 

 rivers that flow into it, for the purpose of depositing 

 their spawn. Such peculiarities of habitat form one of 

 the stumblingblocks of geology ; and it is fortunate that 

 the cultivators of this science are not obliged to place 

 their sole reliance on the palasontological contents of the 

 strata which they wish to investigate, as they have also 

 the mineral composition, as well as the relative juxtaposi- 

 tion, of those strata to guide them in the investigation. 



Zones. It had long been known that different parts 

 of the sea-bed were inhabited by special forms of animal 

 life ; but Risso, the celebrated naturalist of Nice, was 

 the first who proposed its distribution into zones of 

 depth. His theory was derived from observations on the 

 Mediterranean fishes. The late Professor Edward Forbes 

 added much to our knowledge of such distribution ; and 



