184 AXNTVEESARY ADDKESS 



be the case with a species of coral obtained in tbe * Challenger ' 

 Expedition, which ranged from a depth of 30 to one of 2900 

 fathoms, and was very variable in size. 



5. Light. 

 Milton tells us of the 



"world of waters dark and deep." 



One of the most interesting problems relating to the subject of this 

 Lecture is whether the above is a poetical idea or based on fact, as 

 regards the absence of light in the abysses of the ocean. 



"We do not know to what extent the sun's rays penetrate the sea, 

 nor whether the bottom at all depths is absolutely devoid of light. 

 An ingenious apj)aratus, which was contrived by Dr. Siemens for 

 ascertaining the presence of light at different depths by means of 

 highly sensitive photographic paper, has never yet been properly 

 tried. An experiment of this kind, made by Professor Forel, 

 proved that in the Lake of Geneva, even at a depth of only 30 

 fathoms, the paper was entirely unaffected after protracted ex- 

 posure. But the water of that lake is peculiar ; it is said to be 

 rendered less transparent by suspended and floating particles of 

 mica brough.t from glacier streams, and to have thus acquired its 

 deep blue colour. I cannot believe that the only abyssal light, if 

 there be any, is phosphorescent. 



At all events we are certain that, as regards the sea, many 

 animals at very great depths have eyes, and tbat there is no absence 

 of colour. 



Cuttlefishes, which have eyes not less highly organized than our 

 own, have frequently been obtained from depths of many hundred 

 fathoms ; they do not eat phosphorescent polypes and such small 

 deer. Nor are the deep-sea Mollusca blind. During the ' Porcupine ' 

 Expedition of 1869 an undescribed species of Pleurotoma from 2090 

 fathoms had a pair of well-developed eyes on short footstalks ; and 

 a Fustcs from 1207 fathoms had its eyes at the base of the tentacles. 

 The last-named molluscs chiefly prey on bivalves. I have taken at 

 moderate depths, living on the same ground, closely allied species 

 of univalve molluscs, of which some were eyeless or blind, and 

 others were provided with the usual organs of vision. ISTumerous 

 instances have been given by the ' Challenger ' naturalists of 

 apparently seeing as well as of apparently sightless animals taken 

 at great depths. Professor Semper, of Wiirzburg, says, in ' The 

 Natural Conditions of Existence as they affect Animal Life ' 

 (1881): "Many creatures furnished with well-constructed eyes 

 live associated with the actually blind species, and which have 



