374 PRINCIPLES AND METHODS OF PALAEONTOLOGY. 



flojjwhelks, shore crabs, Trliich cover the rocks between high aud low 

 water marks. During calm weather he might imagine that these consti- 

 tuted the chief inhabitants of the sea; but should a heavy gale of wind 

 set in landwards, he is soon undeceived, for the waves, tearing up the 

 sea bottom at depths greater than those which are ordinarily exposed 

 by the recession of the tide, cast on shore vast numbers of new crea- 

 tures, such as whelks, sandstars, corallines, and great masses of sea- 

 weed, with whole colonies of animals attached to them, which habitually 

 remain in the deeper regions. 



Not satisfied with such accidental revelations, modern investigators 

 have systematized and extended the explorations of marine depths by 

 means of the use of the "dredge," a simple apparatus, long used by 

 oyster fishermen to procure their merchandise, aud, of course, equally 

 applicable to the dragging up of other inhabitants of the tloor of the 

 sea. 



It results from a loug series of such observations that at least five 

 zones, each characterized by peculiar forms of animal or vegetable life, 

 may be distinguished at dilierent depths. They are, 1st, the "littoral" 

 zone, corresponding with the interval between high aud low watermarks; 

 I'd, tlie "circumlittoral" zone, extending from low water mark to the 

 lowest limit at which the coral-like plant Nidlipora is found, a depth, in 

 our latitudes, of between fifteen and twenty fathoms; 3d, the "median" 

 zone, characterized by the abundance of Polyzoa and 8ertularidw which 

 it exhibits, and by the i)redomiuauce of carnivorous forms among its 

 Molhisca; it extends in our seas to about fifty fathoms; 4th, the " infra- 

 median," and, 5th, the "abyssal" zones lie beyond this, but can be hardly 

 said at present to be well defined. It is in them that our corals and 

 Brachiopoda flourish. Much attention has of late been paid to the 

 investigation of the deep sea animals and plants, and numerous species 

 have been found at very great depths. As might have been expected, 

 from the greater uniformity of physical conditions at such depths, the 

 same species have been found at very distant localities, and exhibit a 

 wide geographical range in latitude as well as longitude. Another 

 peculiarity more marked even than could have been anticipated is the 

 afiQuity and even identity of many species with tertiary and cretaceous 

 forms. 



The extreme limits of vegetable and of animal life are not known. The 

 higher Algw, such as sea weeds and NulUpora,, are, in our own latitudes, 

 not found below twenty fathoms; but it is not improbable that the 

 JJiatomacecc flourish at the furthest limits of life. 



Both the number of si)ecies and the number of individuals of animals 

 diminish at greater depths. A greater profundity than two hundred 

 fathoms is not to be reached within a very considerable distance of any 

 part of the British coasts; but in both northern and southern seas living 

 animals have been drawn up from more than three hundred fathoms (or 

 1,800 feet) below the surface. It is important to remark that the inhab- 

 itants of these aud still greater depths, however diminished in number, 

 do not appear to become degraded in organization, but consist of Crus- 

 tacea, Echinodcrmata, Gasteropoda, Lamellibranchiata, Polyzoa, and Acti- 

 nozoa, of types quite as elevated as those which are found in more sluillow 

 waters, but they are frequently less brilliantly colored than the latter. 

 While the laws of distribution, as they have been at present determined, 

 therefore, do not enable us to say precisely at what depth living animals 

 can no longer exist, nor even to trace the influence of depth in modify- 

 ing their forms, they seem, nevertheless, to point to certain assemblages 

 as characteristic of certain ranges of depth. For inst.-.nce, limpets and 

 periwinkles appear to be absolutely characteristic of shallow water, being 



