6 



THE NATURAL HISTORY OF 



As it is on the land, so is it in the sea ; not that, 

 as old philosophers fancied, every terrestrial creature 

 has its double beneath the waves, but that the sub- 

 marine population is grouped into geographical pro- 

 vinces, which, though well marked in their more 

 central and most developed portions, are merged at 

 their bounds indistinguishably into the edges of 

 neighbouring realms. These submarine provinces 

 have a more or less distinct correspondence with 

 those of the neighbouring lands, though sometimes 

 they differ very considerably from the latter in their 

 extent, since the physical features which may con- 

 stitute boundaries in the one, may not be sufficiently 

 extended or developed in the other as to impede the 

 spread of peculiar species of animals and plants, or 

 of one or the other only, as the case may be. Ma- 

 rine creatures, owing to their organization and the 

 transporting powers of the element in which they 

 live, are much more capable of diffusion as a whole, 

 than terrestrial ; hence we should expect to find the 

 regions into which they are grouped beneath the 

 waves, of much vaster dimensions than those con- 

 stituted by the geographical assemblages of their 

 terrestrial brethren ; and such is, to a great extent, 

 true. Nevertheless, the inequalities of the sea-bed, 

 the modifications of the temperature of the ocean, 

 produced by currents pouring through it like mighty 

 rivers, and leaving with them the climate — some- 

 times more genial, sometimes more rigorous of the 

 latitudes whence they have derived their source, the 



