DEGENERATION OF EYES 209 



adaptation is not . demonstrated it is really present. But 

 such an assumption belongs to the domain of faith. 



"We ought on the contrary to say : We know that definite 

 stimuli must produce an effect on or in the organism — that 

 they must give rise to definite changes of form, definite 

 characters, whether these be useful to the organism or not. 



When we maintain this we take our stand, not on mere 

 assumptions, but on physiological facts. Normal physiology 

 and pathology in like measure speak for us with the weight 

 of all their fundamental truths. 



Thus there is certainly a physiological basis for the belief 

 that the above -described variations of the sponge -skeleton 

 are simply to be ascribed to changes of external, i.e, of 

 nutritive conditions, of the material composition of the body. 



From this point of view I will permit myself to discuss 

 the several instances which Weismann gives in his latest 

 paper in support of his explanation of the degeneration of 

 disused organs. 



Weismann attributes the degeneration of the eyes in 

 subterranean animals to the cessation of natural selection. 

 Without any doubt, this cause is of very great importance. 

 But with equal certainty physiological considerations lead to 

 the conviction that continual absence of the stimulus of light 

 by itself, without anything else, must gradually injure and 

 finally destroy the capability of the eye to serve as the organ 

 of sight. First of all, the profuse circulation of blood in the eye 

 would be diminished, and thereby the nutrition of the whole 

 be affected ; the muscles, the accommodation apparatus, would 

 become unfit for use ; the retina would be altered. At the 

 same time, the pigment of the eye, which is always connected 

 with the action of light, would disappear, and thus already 

 the eye as such would be rendered almost useless, brought 

 morphologically nearly to the stage of an organ merely 

 susceptible to light, from which it was evolved. 



p 



