44 THE DEPTHS OF THE SEA. [CHAP. I. 
phyton linckii, M. and T., from Mr. Gwyn Jeffreys’ 
dredgings in 1870. Deep-sea forms dredged round 
our coast identical with northern species have been 
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 ease 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. 
