GENERAL INTKODUCTION TO THE ZOOLOGICAL REPORTS. 39 



upon climate and upon facilities for immigration ; and consequently along with the land 

 the zones of shore-life are broken up into regions and provinces, more or less definitely 

 characterised by predominating groups of inhabitants. It is thus allowable and convenient 

 to speak of the littoral fauna of the Australian or of the Indian region, or of the Antillean 

 or Mascarene province. These regions and provinces are somewhat more sharply defined 

 in the littoral and circum-littoral, than they are in the median and infra-median zones, 

 and the distribution of the genera of marine invertebrates which form the fauna of the shore 

 belts, is as a rule much wider and more general than that of the animal forms, vertebrate 

 and invertebrate, which inhabit the contiguous continents and islands. This probably 

 arises from several causes ; conditions which are nearly similar, are much more continuous 

 in the sea than they are on the land, and there are fewer definite barriers to distril^ution ; 

 variations of climate are more extreme, and more immediate in their physiological efiect 

 in air than they are even in shallow water ; and the means and opportunity of dift'usion 

 of aquatic animals are as a rule much greater, seeing that most marine animals pass a 

 longer or shorter period of their lives as minute free-swimming larvae, and while in that 

 condition are borne along and scattered l^y tides and currents. Mr Wallace states that 

 "about forty-eight" out of upwards of eighty "families of marine moUusca are cosmopolitan, 

 ranging over both hemispheres, and in cold as weU as warm seas. About fifteen are 

 restricted to the warmer seas of the globe ; but several of these extend from Norway to 

 New Zealand, a distribution which may be called universal, and only two or three are 

 absolutely confined to tropical seas." i Our information on this matter is still far from 

 complete, but there is little doubt that the generalisation is in the main true, and that 

 it applies with equal or even greater force to other classes of the shallow water fauna. 



In temperate and tropical seas, at a depth of from 400 to 500 fathoms, the number 

 of species begins to increase, and the number of individuals usually rises immensely ; 

 but although many genera which occur in the median and infra-median shore-zones 

 pass down to great depths, the fades of the abyssal fauna is not that of a mere extension 

 of the faima of the shore-belts into deeper water ; it gives rather the efi"ect of a 

 specific fauna deriving a marked individuality from the abundance of certain conspicuous 

 forms which are for the most part special ; and which would appear to have been derived 

 from a genetic source difi"erent from that of the shore fauna. 



The abyssal fauna occupies the floor of the vast lake-like expanse of comparatively 

 still water, which fills the bed of the ocean from depths of 500 or 600 fathoms to the 

 bottom. Throughout the region occupied by the abyssal fauna the physical conditions 

 which have the most immediate influence upon the distribution of animal life are very 

 uniform, their variations occurring within narrow limits. Even at its more moderate 

 depths the temperature is but little affected by direct solar radiation, and is consequently 



' The Geographical Distribution of Animals, by Alfred Russel Wallace, vol. ii. p. 537, London, Maciiiillan & Co., 

 1876. 



