128 
ANNIVERSARY ADDRESS. 
there is to show that the animals with eyes in the dark caves have 
not been there long enough to lose them. "We know, however, 
that the deep-sea fishes have, like nocturnal feeders, large eyes, 
which are adapted to a habitat where there is but little light, and 
that many of the abyssal fauna—including fishes—are luminous on 
account of their mucus being phosphorescent, while others have 
under their control special phosphorescent organs* which may 
enable the sexes to find each other. The brilliant colours of many 
of these deep-sea creatures would at one time have been regarded as 
a proof that the rays of the sun reached their habitat. Experiments 
have, however, shown that pigments in animals as well as in flowers 
are developed in darkness as well as in daylight. 
I cannot close this address without very briefly referring to the 
so-called “ Chromatic function,” with which the eye is closely 
connected. The term is applied to the protective changes of colour 
observed in fishes, reptiles, frogs,f etc., which are due to the altered 
distribution of differently-coloured pigment cells. The action of 
light upon their skin does not produce these phenomena, which 
have been shown to he dependent upon the light reflected from 
external objects reaching the eye. Thus pipe-fish when amongst 
living sea-weeds resemble them even to a mixed colouring of 
green, yellow, and white, and after having rested half an hour 
amidst dead sea-weeds become so dirty-brown that they can hardly 
be distinguished from their immediate surroundings.^ 
The adaptation of the eye to light is the most delicate response 
of nerve to Nature’s touch. "We can investigate the minute anatomy 
of the retina, but, having done so, we find ourselves as far off as 
ever from comprehending how it is that contact with waves of 
ether can produce in us the sensations that we receive from sight. 
This remains always a mystery as obscure as life itself. The eye 
cannot be satisfied with seeing, because the mind recognizes that 
seeing is but the effect of a cause which cannot be seen. 
* See Gunther on “Deep Sea Fishes,” ‘Report ‘Challenger Expedition,’ 
Zool., vol. xxii (1887). 
f See Lister’s paper, “ On the Cutaneous Pigmentary System of the Frog,” 
‘ Philosophical Transactions,’ 1858, p. 627 ; and Pouchet’s “ Note sur la muta- 
bilite de la coloration des Rainettes, etc.,” ‘Academie des Sciences, comptes 
rendus,’ tome xxvi, p. 574. 
+ See “Remerkungen iiher den Farhenwechsel einiger Fische,” by Heinke, 
‘ Schriften des Natiir Wissenschaftlichen Yereins fiir Schleswig-Holstein,’ Kiel, 
1873, p. 225. 
r 
