396 THE STUDY OF ANIMAL LIFE CHAP. 



they are laid. But there is room here for further inquiry. 

 Ogneff found that Axolotls kept in darkness, and starved, 

 lost their black pigment-cells, both inside and outside. 



It has been shown by Ogneff that goldfishes kept in 

 the dark for three years become totally blind, the essential 

 parts of the retina (the image-forming portion of the 

 eye) disappearing. Thus a fish imprisoned in a cave 

 might become blind through the influence of darkness 

 and disuse on the individuals. If the dwindling of the 

 eye was more pronounced in the second and still more 

 in the third generation, the case would show the trans- 

 mission of an individually acquired peculiarity an 

 environmental modification. It is more probable, how- 

 ever, that the blindness of cave animals is due to a 

 germinal variation. It is very interesting to notice 

 Loeb's result that while a certain amount of blindness 

 can be readily induced in embryos of a fish called Fun- 

 dulus heteroclitus (e.g. by fertilising the ova with sper- 

 matozoa from another fish (Menidia), or by adding a little 

 potassium cyanide to the water containing the develop- 

 ing ova, or by exposing the newly fertilised ova to IOAV 

 temperature for some hours), lack of light does not influ- 

 ence the development of the eyes. 



One case of the influence of light seems very instruc- 

 tive. It is well known that flat fishes like flounders, 

 plaice, and soles lie or swim in adult life on one side. 

 This lower side is unpigmented ; the upper side bears 

 black and yellow pigment-containing cells. In part, 

 probably, this is the outcome of a process of sifting con- 

 tinued for ages. For coloration is a very variable char- 

 acter, and it is economical, to say the least, to have no 

 pigment on the surface which is not seen. But it is open 

 to question whether the characteristic is so advantage- 

 ously protective as is usually imagined : thus the coloured 

 upper side in soles is very often covered with a layer of 

 sand. Soles come out most at night, many live at depths 

 at which differences of colour are probably indistinct. 

 In shallower water the advantage is likely to be greater, 

 though the white under-side slightly exposed as the fish 

 rises from the bottom may attract attention disadvan- 



