81 



Although the dredge has long been applied to certain 

 economical purposes, such as the procuring of oysters and 

 marine matter for manure, it is only of late years that atten- 

 tion has been drawn to its great importance as an instrument 

 of scientific research, alike valuable to the geologist and 

 zoologist. Its advantages consist in enabling us to become 

 acquainted with species and forms of animal life not otherwise 

 accessible ; with the circumstances and conditions under 

 which they exist ; and in affording the only correct means of 

 ascertaining the geographical distribution, with the vertical 

 range, as well as local habitat, of the invertebrata inhabiting 

 the bottom of the sea ; also the modifications of form occur- 

 ring invariably in particular species when obtained from cer- 

 tain depths. It will be seen in a moment how the knowledge 

 of this last fact may be applied in enabling the geologist to 

 come to a conclusion as to the depth at which some of the 

 tertiary and more recent formations have been deposited. 



" It is generally known, through the valuable observations 

 of Professor E. Forbes, that in the sea each zone of depth 

 contains its own peculiar species of animal and vegetable life. 

 Of marine animals, some are strictly limited to a particular 

 range of depth; others, and probably the greater number, 

 have their chief development in one region, but are to be 

 found, though more sparingly, throughout the neighbouring 

 regions ; while, again, there are certain species which appear 

 to be not at all particular with respect to depth, but are to be 

 met with in all the four zones into which Forbes has divided 

 the seas of Britain. It appears, at first sight, to be a remark- 

 able fact, that individuals of similar organization should live 

 and flourish indifferently under a pressure of only twice, and 

 under one of sixteen or twenty times that of our atmosphere ; 

 but, when we take into our consideration the comparatively 

 incompressible nature of water, and that the medium inhabited 

 by the dwellers in the abysses of the sea, is not more dense in 

 any appreciable degree than that immediately below the sur- 



