356 On Marine Dredging. 



It is generally known, through the valuable observations of Pro- 

 fessor 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 remarkable fact, 

 that individuals of similar organization should live and flourish in- 

 differently 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 sui-face, it is not easy to imagine in what way the marine 

 mollusca can be affected by pressure. It is evident that they do not 

 require the protection of stronger shells, as these are generally thin- 

 ner and more delicate in deep than in shallow^ water. There are, 

 doubtless, however, other conditions which must render the depths of 

 ocean unfavourable to the development or existence of animal life ; 

 perhaps the most important of these may be the total absence of light ; 

 and the fact, noticed by travellers, of the extraordinary transparency 

 of the Norwegian seas, may explain why mollusca frequent greater 

 depths there than in other parts, as is said to be the case. 



The most general characters of mollusca obtained from a great 

 depth, compared with individuals of the same species inhabiting shal- 

 low water, consist in smallness of size and deficiency in colour. There 

 are also the modifications of form already alluded to, which I brought 

 a few specimens to illustrate — all from the coasts of Zetland- — viz., 

 the common whelk (Buccinum undatum), from less than 10 fathoms, 

 the same from about 25 fathoms, and again, from 60 or 60 fathoms. 

 It will be observed, that the form becomes more elongated the greater 

 the depth of habitat, which appears to b3 a law applying only to the 

 particular species. In Fusus corneus the very reverse occurs, as it 

 is found invariably shorter in proportion to the greater depth it fre- 

 quents. The specimens are from 7, from 30, and from 80 fathoms. 

 I have also brought, for comparison, specimens of two common bi- 

 valves — Venus galina B,nd Venus ovata — obtained in shallow water 

 and at a depth of 80 or more fathoms. 



After further experience, I see no reason to change the opinion 

 I expressed on a former occasion respecting the relation existing be- 

 tween latitude and depth in the sea, not being analogous to that 

 which is found to exist between latitude and elevation on land. 



It is well known that the vegetation of the arctic regions is re- 

 peated at great elevations in temperate and tropical latitudes, either 

 with the same or representative species ; and it has been inferred, 



