Now if this conservative action of natural selection secures the maintenance of the parts and organs of a species at their maximum of perfection, it follows that these will _fall below this maximum as soon as the selection ceases to operate_. And it does cease as soon as an organ ceases to be of use to its species, like the eye to the species of crustacean which descends into the dark depths of our lakes, or to the abyssal zones of the ocean, or into a subterranean cave-system. In this case all selection of individuals ceases as far as the eye is concerned; it has no importance in deciding survival in the struggle for existence, because no individual is at a disadvantage through its inferior eyes, for instance, by being in any way hindered in procuring its food. Those with inferior organs of vision will, _ceteris paribus_, produce as good offspring as those with better eyes, and the consequence of this must be that there will be a general deterioration of eyes, because the bad ones can be transmitted as well as the good, and thus the selection of good eyes is made impossible. The mixture thus arising may be compared to a fine wine to which a litre of vinegar has been added; the whole cask is ruined because the vinegar mingles with every drop of the wine. As deviations from the normal are always occurring in every part of every species, and among them some that lessen the value of the organ, rarely perhaps at first, but after a time in every generation, a sinking of the organ from the highest point of possible perfection becomes inevitable as soon as the organ becomes superfluous. The functional uselessness of the organ must go on increasing the longer it is disused, as will readily be admitted if it be remembered that only the most perfect adaptation of all the separate parts of an organ can maintain its functional capacity, that all the parts of an organ are subject to variation, and that every deviation from the optimum implies a further deterioration of the whole. An eye, for instance, can no longer vary in the direction of 'better' if it has already reached the highest possible point of perfection; every further variation must deteriorate it. Romanes gave expression to this idea, that the cessation of natural selection alone must cause the degeneration of a part, a decade before I did, but neither he nor the scientific world of his time attached great importance to it, and it was forgotten again. This was intelligible enough, for, at that time, the validity of the Lamarckian principle had not been called in question, and therefore the need for some other principle to explain the disappearance of disused parts had not begun to be felt.