CHAP. 1.] INTRODUCTION. 4] 
through many degrees of longitude. but few of latitude. 
Asaclass, however, they prefer a depth rather beyond 
20 fathoms,’ beyond the reach of very violent climatic 
vicissitudes. They are conspicuous things, showing 
usually sufficiently bold specific characters, and thus 
they are less liable to confusion than most other groups. 
They involve in their history and economy several of 
the principal questions discussed in this volume ; while 
giving, therefore, such a brief sketch as the space at 
my disposal and the amount of my present informa- 
tion may permit, of the additions which have been 
made during our dredging cruises to the knowledge 
of the other invertebrate classes, I will use the echi- 
noderms and the protozoa principally for the purpose 
of general illustration. 
Littoral and shallow-water species of animals must 
be much more liable to have their migrations inter- 
rupted by ‘natural barriers,’ such as deep water 
through which they cannot pass, or currents of 
warmer or of colder water; they must likewise be 
much more affected by local circumstances, such as 
extreme differences between summer and winter tem- 
perature ; so that they might be expected to be more 
circumscribed and local in their distribution than the 
denizens of greater depths—and they certainly are so. 
The conditions of the bottom in the zone from 20 to 
50 fathoms are much more equable than near the 
surface. Direct solar radiation in temperate regions 
affects this zone very slightly, so that it probably 
1 Distribution of Marine Life. By Professor Edward Forbes, F.R.S., 
President of the Geological Society. (From the Physical Atlas of 
Natural Phenomena, by Alexander Keith Johnston, F.R.G.S., &c. 
(Edinburgh, 1854.) 
