THE EYE-LIKE SPOTS IN FISHES. 
233 
other contents than fatty or oily matter,* and the cases are rare 
in which the part which is phosphorescent has not some other 
and more important function also ; where this is not so — as in 
the instance of the phosphorescent insects — we find, and it is 
very curious to note this, that the distinguished histologist 
Kolliker of Wurzburg suggested, nearly a quarter of a century 
ago, that the luminous organs are essentially nervous and pre- 
sent the nearest resemblance to the electrical organs of fishes. 
This point is not noticed by Ley dig, but it would be curious 
if it should finally turn out that the organs he has been 
describing with such care, are intermediate between the 
other two sets. Even if not phosphorescent, it still remains 
possible that these creatures add to the feeble light of great 
ocean-depths by reflecting the light that falls on these eye- 
like organs. We all know how a cat’s eye ‘shines’ in the 
dark, and most of us, it is to be hoped, know that this is no 
case of phosphorescence, but that there is a thick layer of wavy 
fibrous tissue lining the inner face of the eye which reflects the 
light ; reflecting it, by the way, not for the purpose of illumi- 
nating the neighbourhood of a nocturnal beast of prey, but 
reflecting it for the purpose of as many rays as possible passing 
to the retina. In some of the fishes of which we have made 
mention, a somewhat similar tapetum has been found, and it is 
quite possible that that layer does act as a reflector in one of 
the worst illuminated regions of the habitable globe. 
For ourselves, we are inclined to think that the relations of 
these organs to those with electric properties in other fishes is the 
most reasonable that has yet been advanced ; they may have a 
secondary illuminating function of some kind, but it was not the 
necessity for this that first gave rise to their formation. If we 
assume the correctness of Prof. Leydig’s explanation, we have 
before us a very interesting example of the way in which 
scientific hypotheses become theories. An obvious external 
resemblance to secondary eyes led the first naturalist who 
studied them closely to look on them as accessory organs of 
sight ; a closer and more detailed investigation has shown that 
this resemblance is purely artificial, and an appeal is now made 
not to a seeming likeness, but to a more deep-seated similarity, — 
to one that depends not on the position of rounded fibrous bodies 
or to a peculiar arrangement of pigment ; but to one, in fine, that 
depends on community of structure, the first and best imple- 
ment in any investigation into living objects, and the one that, 
side by side with the history of development, is the only 
* On this point the student is especially to be referred to the various 
papers of the late Prof. Panceri ; for the phosphorescence of fishes, consult 
‘ Panceri ’ in Zool. Record for the year 1871. 
