CHAP. I.] INTRODUCTION. 41 



through many degrees of loBgitude, but few of latitude. 

 As a class, 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 



' Distribution of Marine Life. By Professor Edward Forbes, F.K.S., 

 President of the Geological Society. (From tlie Physical Atlas of 

 Natural Phenomena, by Alexander Keith Johnston, F.E.G.S., &c. 

 Edinburgh, 1854.) 



