27 



observations with figures, and thus to supply good evidence that my 

 original statements were correct in every particular, though necessarily 

 somewhat incomplete. 



5. An Experiment concerning the Absence of Color 

 from the lower Sides of Flat-fishes. 



By J. T. Cunningham, M.A., Naturalist to the Marine Biological Association. 



eingeg. 1. December 1890. 



One of the most interesting questions which biological research 

 has still to decide is whether adaptations in organisms are due to the 

 natural selection of indefinite variations or to the definite influence of 

 the conditions of life. One school of evolutionists, that of which 

 Weismann is one of the most eminent leaders, maintains that every 

 character in animals is an adaptation and every adaptation is sufficiently 

 explained by indefinite variation and natural selection. Another school 

 believes that many things are not adaptations and that those characters 

 which are adapted are due to the definite influence of conditions. The 

 former school would I suppose maintain that the whiteness of the lower 

 sides of flat-fishes was an adaptation, and was due to selection. What 

 is the especial advantage of this character to flat-fishes I am unable to 

 perceive. But it seems to me more probable that it is due in some way 

 to the fact that little or no light can fall on the lower sides of these 

 fishes, because these sides are generally in contact with the ground. 



The following experiment seems to me to support very strongly 

 the latter views ; it was carried out in the Plymouth Laboratory of the 

 Marine Biological Association. 



At the beginning of last May I received from Mevagissey in Corn- 

 wall a large number of young flounders {Pleuronectes flesus) in process 

 of metamorphosis. They were very transparent and measured 11-5 to 

 12-7 mm in length. In a few the metamorphosis was almost complete 

 the left eye having reached the edge of the head but in the majority 

 the left eye though it had commenced its »migration« was still on the 

 lower side. The little fish had already developed the habit of lying on 

 the bottom on the left side. Nearly all the pigment, i. e. the chromato- 

 phores had disappeared from the lower side, where only a few scattered 

 black and yellow cells remained : on the upper side the pigmentation 

 was considerable, but not so fully developed as in the adult. 



On May 8th I took about 15 or 16 of these small flounders and 

 placed them in a glass vessel without sand. This vessel I placed on a 

 plate of glass supported at the ends by two supports. Beneath the glass 

 plate I arranged a mirror about 15 inches by 12, sloping it at an angle 



