Adaptation to Environment 321 



caves also might have abnormal eyes even if they had 

 never lived in caves. 



Through these facts the old idea becomes question- 

 able, namely, that the cave animals had originally 

 been animals with normal eyes which owing to disuse 

 had undergone a gradual hereditary degeneration. 



Recent experiments made on the embryos of the fish 

 Fundulus have yielded the result that it is possible to 

 produce blindness in fish by various means other than 

 lack of light. x Thus the writer found that by crossing 

 the egg of Fundulus with the sperm of a widely different 

 species, namely, Menidia, blind embryos were produced 

 very frequently; that is to say such embryos had the 

 degenerate eyes characteristic of blind cave fishes. 

 Very often no other external trace of an eye, except a 

 gathering of pigment, could be found, while a close 

 histological examination would possibly have resulted 

 in the demonstration of rudiments of a lens and other 

 tissues of the eye. 



Another method of producing blind fish embryos 

 consists in exposing the egg immediately, or soon after 

 fertilization, to a temperature between o and 2 C. 

 for a number of hours. Many embryos are killed by 

 this treatment, but those which survive behave very 

 much like the hybrids between Fundulus and Menidia, 

 i. e., a number of them have quite degenerated eyes. 

 If the eggs have once formed an embryo they can be 



x _Loeb, J., Biol. Bull, 1915, xxix., 50. 



21 



