20 



THE NATURAL HISTORY OF 



numerous, though, as if in compensation, the num- 

 ber of individuals is so great as to prevent incon- 

 venience from the comparative scarcity of kinds. 

 In both vertebrate and invertebrate divisions of the 

 animal kingdom this is manifest. We may exem- 

 plify the fact by reference to the best investigated 

 sections of each. Thus, whilst the number of 

 generic types of fishes in the Mediterranean region 

 is 227, in the British seas we have only 130, and in 

 Scandinavia there is a still further decrease to 120 ; 

 of mollusca, in the first-named region there are 155 

 genera, in the second 129, and in the third 116. 

 The number of species of fishes in the Mediterra- 

 nean seas is 444 ; in the British seas 216 ; in the 

 Scandinavian seas 170; and of marine mollusca 

 (exclusive of Nudibranchiata and Tunicata, data for 

 computing which tribes are insufficient), Mediter- 

 ranean 600, British 400, and Scandinavian 300. 



But climate alone is not the only cause of change 

 in our course from north to south. The changes 

 in the geological structure of the European shores 

 are frequent and striking in that direction, and 

 affect materially, or rather determine the physical 

 aspect of the coasts, and the conformation of 

 the neighbouring sea-bed. Geologists have to deal 

 not merely with the land as it is exhibited above 

 water ; they must prosecute their science in the 

 recesses of the sea, and trace in the depths and 

 shallows of its floor the continuations of the plains 

 and hills, and valleys of the contiguous lands, and 



