40 



THE VOYAGE OF H.M.S. CHALLENGEE. 



almost uniform throiiglioiit the year. At depths of from 800 to 1200 fathoms the 

 constant temperati;re may average about 40° F., and at depths of 2000 fathoms and 



upwards the thermometer usually indicates a mini- 

 mum of 35°, while in certain restricted areas only 

 it sinks to a degree or so below the freezing-point. 

 The range of the thermometer throughout the whole 

 of the abyssal region does not, therefore, as a rule 

 exceed 6° or 8° F. As we pass into higher latitudes, 

 north and south, the temperature-conditions of the 

 abyssal region gradually rise towards the surface, 

 until, in the seas of Northern Scandinavia and 

 Labrador, the temperature of the whole mass of 

 water, from 100 fathoms downwards, has a mean 

 temperature of about 36° F. Of all circumstances, 

 a uniformity of temperature seems to favour most 

 the extension of animal species, so that while maiy 

 forms characteristic of the infra-median zone occur 

 in the lists of the Scandinavian naturalists, from 

 depths of from 100 to 200 fathoms, there is like- 

 wise a general extension upwards of the Gorgonise, 

 the Isids, the Corals, the Elpididae, and other groups 

 belonging to the true abyssal fauna ; and, still further 

 north, abyssal forms, as, for example, Bhizocrinus 

 among the Lofoten Islands, follow the cold water 

 almost to the shore. 



So far as we can judge, direct sunlight does 

 not penetrate to great depths, and consequently 

 in deep-sea animals the eyes are often absent, or 

 they are atrophied by disuse ; or, as in the case of 

 many crustaceans, the organs in the position of the 

 eyes are modified to perform some other function- 

 In some cases, at moderate depths, where a certain 

 amount of light may still be supposed to penetrate, 

 the eyes are large and clear, exaggerated apparently 

 to catch its last feeble rays. Many deep-sea animals 

 are slightly, and some are vividly, phosphorescent. 

 It is difficult to imagine what the object or the 

 effect of this faint illumination may be; but, at all events, the light does not seem to be 

 sufficiently stimulating to maintain organs of vision in a state of functional activity. 



Fig. 17. — Rhizocrinus lofotensis, M. Sars. 



