118 GEOGEAPHICAL DISTEIBUTIOK. 



vertical distribution is light. It is claimed in confinnation of this 

 view that the limit of light-penetration in the oceanic waters, as fixed 

 by Secchi, PourtalSs, and Bouguer, corresponds closely with the 

 forty to fifty fathom line, marking the upper boundary of the deep- 

 sea fauna, or, more strictly, the line separating the littoral from the 

 deep-sea fauna. Below this line, therefore, the fauna is one of dark- 

 ness, and above it, except in so far as certain animal groups may be 

 nocturnal in their habits, one of light. In support of this proposi- 

 tion Professor Fuchs emphasises the fact that, " with their character 

 of animals of darkness, numerous peculiarities in the organisation 

 and nature of the deep-sea animals agree. Thus it is known that very 

 many deep-sea animals either have uncommonly large eyes, after 

 the fashion of nocturnal animals, or are completely blind ; it is also 

 well known that they are, for the most part, either pale and colour- 

 less, or unicolourous, and that varied colouration is exceedingly sel- 

 dom met with among them; and, finally, it is likewise well known 

 that a very large proportion of deep-sea animals, in many groups, 

 indeed the majority, are vividly luminous. This last peculiarity is 

 of special importance, for it is clear that luminosity can be of con- 

 sequence only to such animals as are destined to live in darkness, 

 and, in point of fact, scarcely any luminous animals are known to 

 us from the littoral region." 



While it may, perhaps, be admitted that temperature is not the 

 only, or even principal, agent in determining distribution, it must, 

 nevertheless, be confessed that certain grave objections present 

 themselves to the theory which looks upon light as the determin- 

 ing factor ; indeed, the objections are much of the same kind as 

 those which have been urged against the thermal theory. If the 

 fact is surprising that corals do not descend below the one hundred 

 and twenty foot line, when the temperature for a very considerable 

 distance beyond that point is still above the normal required by 

 them, is it not perhaps equally surprising that they should be 

 limited at this point at all, seeing that the penetration of light ex- 

 tends to fully double or treble the depth, or, as has been more re- 

 cently shown by MM. Fol and Sarasin in the case of the Mediterra- 

 nean Sea, to even ten times that depth ? Can it be rationally 

 conceived that such lowly organisms, devoid of special visual or- 

 gans, can be so affected by the conditions of light and darkness as 

 not to be able to endure that amoimt of obscurity which-distin- 



