F. R. Coivper Reed — Blind TriloUtes. 557 



and the change of habitat too rapid for adaptation of the organ to 

 the new conditions, it degenerated and atrophied. There may be 

 little truth in Verrill's view that more or less sunlight penetrates 

 to the greatest depths, giving an illumination at 2,000 to 3,000 

 fathoms perhaps equal to our partially moonlight nights, and possibly 

 at greater depths equal to starlight ; but McCulloch and Coldstream's 

 clever suggestion that the light of phosphorescent organisms supplies 

 the place of sunlight in the deep sea has much probability in it when 

 we consider the great development of phosphorescent organs in 

 animals at abyssal depths. Hickson ^ remarks that phosphorescence 

 is only locally distributed, and that in some regions the darkness is 

 so absolute that it can only be compared" with the darkness of the 

 great caves. The fact that the fauna of the deep sea does not entirely 

 consist of blind or large-eyed forms is partly explicable on this view, 

 but is probably to a larger extent due to the immigration of species 

 with normally developed eyes which has always been taking place 

 from the shallow-water regions. It is to be remembered that the 

 majority of modern deep-sea animals are merely modified types of 

 shallow-water forms, and that there are few which can be considered 

 ancestral in character. Thus, even if other considerations already 

 discussed allowed us to consider the blind trilobites belonging to our 

 ' Primitive ' group as members of a deep-sea adapted fauna, the 

 abundance and large proportion of such ancestral types would be 

 contrary to modern analogy. Many characteristic modern deejD-sea 

 forms also may occasionally wander into shallower regions where 

 faint rays of sunlight penetrate, and the young stages of others may 

 be passed at or near the surface of the sea.- 



It is not, therefore, very surprising that very few animals 

 belonging to families usually provided with eyes are completely 

 blind. Hickson remarks that the large-eyed forms preponderate 

 from 300-600 fathoms, which includes the region which rays of 

 sunlight faintly penetrate. But in water of over 1,000 fathoms 

 small-eyed and blind forms are in a majority, for it is below the 

 sunlight limit, and eyes are therefore for the most part useless. 



Agassiz^ has said that it is difficult to draw any conclusions from 

 the great diversity presented by the organs of sight in the Crustacea, 

 and that one cannot help being struck by the small number of 

 deep-sea Crustacea which have lost their eyes. It is well to bear in 

 mind his caution^ as to "drawing conclusions with reference to 

 physical conditions derived from organs of sense which may serve 

 other purposes than that of vision alone." 



It must also be remembered that there are many blind Crustacea 

 and other marine invertebrates either without eyes or with only 

 rudimentary organs of sight which have been dredged from 

 comparatively shallow water. From the habits of these forms it 



1 " Fauna of the Deep Sea" (Mod. Sci. Series), pp. 22-28. ^^'g]g 



^ Hickson, loc. cit. 



shales 



3 Three Cruises of the "Blake," vol. ii, p. 44. See also W. Matihall. •=" 

 Tiefsee und ihr Tierleben " (Leipzig, 1888), p. 260, etc. jxamined 



* Ibid., vol. 1, p. 308. ,ch, F.E.s'. 



36 



