The Lamarckian view--that through disuse the organ is diminished during the life of the individual, in part, at least, on account of the diminution of the amount of blood going to a resting organ, and that this effect is transmitted to succeeding generations--not only would theoretically account for unlimited progressive degeneration, but is the only view so far examined that does not on the face of it present serious objections. Is this theory applicable in detail to the conditions found in the Amblyopsidæ? Before going further, objections may again be raised against the universal assumption that the cessation of use and the consequent panmixia was a sudden process. This assumes that the caves were peopled by a catastrophe. But it is absolutely certain that the caves were not so peopled, that the cessation of use was gradual, and the cessation of selection must also have been a gradual process. There must have been ever-widening bounds within which the variation of the eye would not subject the possessor to elimination. Chologaster is in a stage of panmixia as far as the eye is concerned. It is true the eye is still functional, but that the fish can do without its use is evident by its general habit and by the fact that it sometimes lives in caves. The present conditions have apparently existed for countless generations--as long as the present habits have existed--and yet the eye still maintains a higher degree of structure than reverse selection, if operative, would lead us to expect, and a lower than the birth mean of fishes depending on their eyes, the condition that the state of panmixia alone would lead us to expect. There is a staying quality about the eye with the degeneration, and this can only be explained by the degree of use to which the eye is subjected. The results in Chologaster are due to panmixia and the limited degree of use to which the eye is put. Chologaster Agassizii shows the rapid diminution with total disuse. The difference in the conditions between Chologaster and Amblyopsis, Typhlichthys and Troglichthys, is that in the former the eyes are still in use, except when living in caves; in the latter they have not been in a position to be used for hundreds of generations. The transition between conditions of possible use and absolute disuse may have been rapid with each individual after permanently entering a cave. Panmixia, as regards the minute eye, continued. Reversed selection, for economy, can not have affected the eye for reasons already stated. The mere loss of the force of heredity, unless this was caused by disuse, or the process of germinal selection, can not have brought about the conditions, because some parts have been affected more than others. [Illustration: FIG. 7.--Head of a very young _Amblyopsis_ before all the yolk is absorbed.]