CHAP. T. | INTRODUCTION. 39 
and yet the geographical distribution of most of the 
shallow-water species is well defined, and frequently 
somewhat restricted. Unfortunately we know as yet 
very little about the general distribution of marine 
animals. Except along the coasts of Britain and 
Scandinavia, a part of the North American coast, and 
a part of the Mediterranean, we know absolutely 
nothing beyond the shore zone, or at all events beyond 
10 or 15 fathoms. What little we do know is con- 
fined almost entirely to the mollusca, and is due, not 
so much to scientific research as to the commercial 
value which the acquisitive zeal of conchologists has 
placed upon rare shells. It may be supposed, how- 
ever, that the same laws which regulate the distri- 
bution of littoral and sub-littoral mollusca, affect in 
hke manner that of shallow-water annulosa, echino- 
derms, and coelenterates ; indeed, from the scattered 
cbservations which have been made on the distribu- 
tion of these latter groups, it seems certain that such 
is the case. 
Woodward' regarded the marine mollusca as oceupy- 
ing eighteen well-defined ‘provinces,’ fulfilling more 
or less completely the condition of having at least one- 
half of the species peculiar to the province. Edward 
Forbes defined twenty-five such ‘regions ;’ but it must 
be remembered that in both cases at least three-fourths 
of the number of areas defined were based upon the 
most imperfect knowledge of the larger and more con- 
spicuous shore shells only. It has been constantly 
observed in the few cases confined entirely to the 
shores of the North Atlantic and the Mediterranean, 
1 A Manual of the Mollusca. By S. P. Woodward. London, 1851. 
P, 354, 
