44 THE DEPTHS OF THE SEA. [chap. i. 



jjhyton linckli, M. and T , from Mr. Gwyn Jeffreys' 

 dredgings in 1870. Deep-sea forms dredged round 

 our coast identical with northern species have heen 

 usually regarded as 'boreal outliers' (Forbes), or at 

 all events as species which have extended their dis- 

 tribution from northern centres. This idea probably 

 arose in a great measure from their having been 

 discovered and first described in Scandinavia. We 

 actually know nothing about their centres of distri- 

 bution ; all we know of them is that they are the in- 

 habitants of an enormously extended zone of special 

 thermal conditions, which ' crops out,' as it were, or 

 rather comes within range of the ordinary means of 

 observation, off the coasts of Scandinavia. 



Edward Forbes pointed out long ago the kind of in- 

 verted analogy which exists between the distribution of 

 land animals and plants and that of the fauna and flora 

 of the sea. In the case of the land, while at the level 

 of the sea there is, in temperate and tropical regions, a 

 luxuriant vegetation with a correspondingly numerous 

 fauna, as we ascend the slope of a mountain range 

 the conditions gradually become more severe ; species 

 after species belonging to the more fortunate plains 

 beneath disappear, and are replaced by others whose 

 representatives are only to be found on other moun- 

 tain ridges, or on the shores of an arctic sea. In the 

 ocean, on the other hand, there is along the shore line 

 and within the first few fathoms, a rich and varied 

 flora and fauna, which participates and sympathises 

 in all the circumstances of climate which affect the 

 inhabitants of the land. As we descend, the condi- 

 tions gradually become more rigorous, the tempera- 

 ture falls, and alterations of temperature are less felt. 



