Morgan’s comment on these elaborate speculations of Darwin is very caustic and concise. Referring to factorial mutations, which give rise to races of flies having _supernumerary_ and _vestigial_ organs, he says: “In contrast to the last case, where a character is doubled, is the next one in which the eyes are lost. This change took place at a single step. All the flies of this stock, however, cannot be said to be eyeless, since many of them show pieces of eye—indeed the variation is so wide that the eye may even appear like a normal eye unless carefully examined. Formerly we were taught that eyeless animals arose in caves. This case shows that they may also arise suddenly in glass milk bottles, by a change in a single factor. “I may recall in this connection that wingless flies also arose in our cultures by a single mutation. We used to be told that wingless insects occurred on desert islands because those insects that had the best developed wings were blown out to sea. Whether this is true or not, I will not pretend to say, but at any rate wingless insects may also arise, not through a slow process of elimination, but at a single step.” (“A Critique of the Theory of Evolution,” 1916, pp. 66, 67.) In directing attention to the fact that a permanent and inheritable reduction of organs to the vestigial state can result from mutation, we do not, of course, intend to exclude the possible occurrence of somatic atrophy due to lack of exercise rather than to germinal change. Thus the blind species of animals in caves may, in some instances, be persistently blind, because of the persistent darkness of the environment in which they live, and not by reason of any inherited factor for blindness. Darwin gives one such instance, namely, that of the cave rat _Neotoma_. To test such cases, the blind animals would have to be bred in an illuminated environment. If, under this condition, they failed to develop normal eyes, the blindness would be due to a germinal factor, and would be inherited in an illumined, no less than a dark, environment.