Adaptation to Environment 327 



eyes can survive in a cave while they can only excep- 

 tionally survive in the open. The cause of the de- 

 generation is a disturbance in the circulation and 

 nutrition of the eye, which is as a rule independent of 

 the presence or absence of light. 



We may by w r ay of a digression stop for a moment to 

 consider the most astonishing and uncanny case of 

 adaptation; namely, the formation of the transparent 

 refractive media, especially the lens in front of the 

 retina. It is due to these media that the rays which 

 are sent out by a luminous point can be united to an 

 image point on the retina. One part of this process is 

 understood ; namely, the formation of a lens. Wherever 

 the optic cup of the embryo is transplanted under the 

 epithelium the latter will be transformed into a trans- 

 parent lens. When the upper edge of the iris is in- 

 jured in the salamander so that the cells can multiply, 

 the mass of newly formed cells also becomes transparent 

 and a lens is formed. This indicates the existence of 

 a substance in the optic cup which makes the epithelial 

 cells transparent; and which also limits the size of the 

 lens which is formed. The lens is not always a perfect 

 optical instrument, on the contrary, it is as a rule 

 somewhat defective. Of course, a great many details 

 concerning the process of lens regeneration have still 

 to be worked out. 



3. It is well known that most marine animals die if 

 put into fresh water and vice versa; and in salt lakes or 



