40 GENETICS AND EUGENICS 



overcast. This signifies to the Lamarckian that the effects of 

 the sun's rays on the human skin are inherited; but to the 

 selectionist it means only that men vary in depth of pig- 

 mentation and that each race has migrated to that climate 

 which it is best fitted to endure. 



As regards the origin of cave animals the same diversity of 

 opinion exists. Some consider that animals which found their 

 way into caves lost their pigmentation and transmitted this 

 condition to their offspring; others hold that such animals 

 as were able to survive when by chance they made their way 

 into caves were probably animals with little pigmentation, 

 which could not very well exist elsewhere. 



As regards the vision of cave animals, the Lamarckians 

 hold that the eyes have degenerated because no longer used, 

 whereas the selectionists hold that the animals which have 

 taken to living in caves have been driven to this course by 

 the degeneration of their eyes, and they point out that the 

 nearest relatives of cave animals are those with poorly 

 developed eyes, which live in semi-darkness. 



Kammerer, very commendably, has put these alternative 

 views to an experimental test. He has reared in daylight the 

 young of the cave salamander, Proteus anguinius. Under 

 these circumstances the skin became pigmented and the eye 

 did not degenerate, as normally; but if the animals were kept 

 in strong light continuously the skin became so heavily pig- 

 mented, including that in front of the eye where the trans- 

 parent cornea forms in ordinary animals living in the light, 

 that in consequence the eye itself degenerated. To overcome 

 this difficulty Kammerer kept the animals in red light, which 

 is less favorable than daylight to pigment formation, but 

 suffices nevertheless to stimulate the eyes to development. 

 The red-light treatment was given for one week out of three 

 during the first eighteen months of the animals' lives. In 

 this way the eye, which in cave-inhabiting individuals is very 

 small and rudimentary, was brought to full development, 

 with a transparent cornea and all other parts necessary for 

 vision. 





