556 F. E. Coivper Reed — Blind TriloUtes. 



exception in the genus to which they belong, and their phylogenetio 

 rank (with the exception of Harpes) demands that compound eyes 

 should be present. We must seek therefore for the special conditions 

 which lead to the loss of eyesight. 



In the case of modern marine animals it is found that a large 

 number of those inhabiting the abysses of the ocean, to which few 

 or no rays of sunlight can penetrate, are either devoid of eyes or 

 have these organs enormously developed, while closely allied species 

 or genera living in well-illuminated zones of the sea have normal 

 eyes. From the analogy of blind cave-animals it is generally 

 believed that this state of things is due directly to the absence or 

 feebleness of the light at the bottom of the deep sea. Packard^ 

 discusses the question and says that "it is most probable that the 

 causes of atrophy or blindness under one set of conditions [i.e. in 

 caves] are the same or nearly the same as in the otlier." But this 

 is questionable, and, apart from the action of other factors, such as 

 pressure and equable temperature, which operate in the deep sea, the 

 facts gathered by Professor S. J. Smith '^ from the dredgings of the 

 " Albatross " indicate " some difference in the conditions as to light in 

 caverns and in the abysses of the ocean, and make it appear probable 

 that in spite of the objections of the physicists some kind of luminous 

 vibrations do penetrate to depths exceeding even 2,000 fathoms." 

 If we exclude shallow-water species, no definite relation has been 

 traced between the amount of modification of the eyes and the depth 

 which the species inhabit. The experiments of f orel, Asper, Fol, 

 and Sarasin, on the dej)th to which the rays of sunlight penetrate 

 which affect photographic plates, were not made in mid-ocean, where 

 the purity and transparency of the water is said to be much greater 

 than near shore or in lakes ; and, moreover, we are not bound to 

 assume that the limits of luminous perception are the same for the 

 retina and visual nerves of all the lower animals. It is, however, 

 proved that " although some abyssal species do have well-developed 

 eyes, there can be no question that there is a tendency towards very 

 radical modification or obliteration of the normal visual organs in 

 species inhabiting deep water" (Packard). The amount of modi- 

 fication appears to depend to some extent on the length of the period 

 during which the organism has inhabited the deep water. Thus 

 Pentacheles, which belongs to a group believed to have frequented 

 deep water for considerable geological periods, has highly modified 

 eyes, while the species more closely allied to shallow-water forms 

 have less modified or only partially reduced eyes, suggesting a more 

 recent immigration into the abyssal regions. In the case of those deep- 

 sea fishes, as Lendenfeld has said, in which the eye was originally 

 strong and well constructed, and the migration into deep water 

 i>'radual and spread over many generations, the eye became gradually 

 *^n.larged ; but in those forms in which the eye was originally weak 



oortvQi 



blind '^^"^^ Fauna of North America": jSTat. Acad. Sci., vol. iv (1886), Mem. 1, 



^ -137. 

 ^^ ^ In. Mag. Nat. Hist., 1886, pp. 194-7. 



