When writing this passage in 1864, I never dreamt that a quarter of a century later, the supposable cause of degeneration here examined and excluded as impossible, would be enunciated as not only a cause, but the cause, and the sole cause. This, however, has happened. Weismann's theory of degeneration by panmixia, is that when an organ previously maintained of the needful size by natural selection, is no longer maintained at that size, because it has become useless (or because a smaller size is equally useful), it results that among the variations in the size, which take place from generation to generation, the smaller will be preserved continually, and that so the part will decrease. And this is concluded without asking whether the economy in nutrition achieved by the smaller variation, will sensibly affect the survival of the individual, and the multiplication of its stirp. To make clear his hypothesis, and to prepare the way for criticism, let me quote the example he himself gives when contrasting the alleged efficiency of dwindling by panmixia with the alleged inefficiency of dwindling from disuse. This example is furnished him by the Proteus.
Concerning the "blind fish and amphibia" found in dark places, which have but rudimentary eyes "hidden under the skin," he argues that "it is difficult to reconcile the facts of the case with the ordinary theory that the eyes of these animals have simply degenerated through disuse." After giving instances of rapid degeneration of disused organs, he argues that if "the effects of disuse are so striking in a single life, we should certainly expect, if such effects can be transmitted, that all traces of an eye would soon disappear from a species which lives in the dark." Doubtless this is a reasonable conclusion. To explain the facts on the hypothesis that acquired characters are inheritable seems very difficult. One possible explanation may indeed be named. It appears to be a general law of organization that structures are stable in proportion to their antiquity; that while organs of relatively modern origin have but a comparatively superficial root in the constitution, and readily disappear if the conditions do not favor their maintenance, organs of ancient origin have deep-seated roots in the constitution, and do not readily disappear. Having been early elements in the type, and having continued to be reproduced as parts of it during a period extending throughout many geological epochs, they are comparatively persistent. Now the