Through these facts the old idea becomes ques­tionable, 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.[276] 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 0° 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 kept at the temperature of 0° for a month or more without giving rise to blind animals. Occasionally such rudimentary eyes were also observed when eggs were kept in a solu­tion containing a trace of KCN. Stockard has succeeded in producing cyclopean eyes in Fundulus by adding an excess of magnesium salt to the sea water in which the eggs developed or by adding alcohol, and McClendon has confirmed and added to these results.

The writer tried repeatedly, but in vain, to produce Fundulus with deficient eyes by keeping the embryos in the dark. Sperm and egg were not allowed to be exposed to the light yet the embryos without excep­tion had normal eyes.

F. Payne raised sixty-nine successive genera­tions of a fly Drosophila in the dark, but the eyes and the reac­tion of the insects to light remained perfectly normal.

Uhlenhuth has recently demonstrated in a very striking way that the development of the eyes does not depend upon the influence of light or upon the eyes func­tioning. He transplanted the eyes of young salamanders into different parts of their bodies where they were no longer connected with the optic nerves. The eyes after transplanta­tion underwent a degenera­tion which was followed by a complete regenera­tion. He showed that this regenera­tion took place in complete darkness and that the transplanted eyes remained normal in salamanders kept in the dark for fifteen months. Hence the eyes which were no longer in connec­tion with the central nervous system, which had received no light, and could not have func­tioned, regenerated and remained normal. The degenera­tion which took place in the eyes immediately after being transplanted was apparently due to the interrup­tion of the circula­tion in the eye, and the regenera­tion commenced in all probability with the re-establishment of the circula­tion in the transplanted organ.

In our own experi­ments it can be shown that the circula­tion in the embryo was deficient in all cases where the eyes degenerated. The hybrids between Fundulus and Menidia have often a beating heart but rarely a circula­tion (although they form blood); and the same phenomenon occurred in the embryos which were exposed to a low temperature at an early period of their lives. Hence all the facts agree that condi­tions which lead to an abnormal circula­tion (and consequently also to an abnormal or inadequate nutri­tion of the embryonic eye) may prevent development and lead to the forma­tion of blind fishes. Eigenmann states that no blood-vessels enter the eye of the blind cave salamander Typhlotriton. The presence or absence of light does not usually interfere with the circula­tion or nutri­tion of the embryonic eye, and hence does not as a rule lead to the forma­tion of degenerated eyes.

This would lead us to the assump­tion that the blind fish owe their deficiency not to lack of light but to a condi­tion which interferes with the circula­tion in the embryonic eye. Such a condi­tion might be brought about by an anomaly in the germ plasm or in one chromo­some, the nature and cause of which we are not able to determine at present; but which, since it occurs in the germ plasm or the chromo­somes, must be hereditary. This would explain why it is, that animals with perfect eyes may occur in caves and why perfectly blind animals may occur in the open. It leaves, however, one point unexplained; namely, the greater frequency of blind species in caves or in the dark and the relative scarcity of such forms in the open.

Eigenmann has shown that all those forms which live in caves were adapted to life in the dark before they entered the cave.[277] These animals are all negatively helio­tropic and positively stereotropic, and with these tropisms they would be forced to enter a cave whenever they are put at the entrance. Even those among the Amblyopsidæ which live in the open have the tropisms of the cave dweller. This eliminates the idea that the cave adapted the animals for the life in the dark.

Only those animals can thrive in caves which for their feeding and mating do not depend upon visual mechanisms; and conversely, animals which are not provided with visual mechanisms can hold their own in the open, where they meet the competi­tion of animals which can see, only under excep­tional condi­tions. This seems to account for the fact that in caves blind species are comparatively more prevalent than in the open.