eye answers to this description as being a very early organ. But waiving possible interpretations, let us admit that here is a difficulty—a difficulty like countless others which the phenomena of evolution present, as, for instance, the acquirement of such a habit as that of the Vanessa larva, hanging itself up by the tail and then changing into a chrysalis which usurps its place—a difficulty which, along with multitudes, has to await future solution, if any can be found. Let it be granted, I say, that here is a serious obstacle in the way of the hypothesis; and now let us turn to the alternative hypothesis, and observe whether it is not met by difficulties which are much more serious. Weismann writes:
"Hence there is no reason to wonder at the extent to which the degeneration of the eye has been already carried in the Proteus, even if we assume that it is merely due to the cessation of the conserving influence of natural selection."But it is unnecessary to depend upon this assumption alone, for when a useless organ degenerates, there are also other factors which demand consideration—namely, the higher development of other organs which compensate for the loss of the degenerating structure, or the increase in size of adjacent parts. If these newer developments are of advantage to the species, they finally come to take the place of the organ which natural selection has failed to preserve at its point of highest perfection."
On these paragraphs let me first remark that one cause is multiplied into two. The cause is stated in the abstract, and it is then re-stated in the concrete, as though it were another cause. Manifestly, if by decrease of the eye an economy of nutriment is achieved, it is implied that the economized nutriment is turned to some advantageous purpose or other; and to specify that the nutriment is used for the further development of compensating organs, simply changes the indefinite statement of advantage into a definite statement of advantage. There are not two
- While the proof of this article is in hand, I learn that the Proteus is not quite blind, and that its eyes have a use. It seems that when the underground streams it inhabits are unusually swollen, some individuals of the species are carried out of the caverns into the open (being then sometimes captured). It is also said that the creature shuns the light; this trait being, I presume, observed when it is in captivity. Now obviously, among individuals carried out into the open, those which remain visible are apt to be carried off by enemies; whereas, those which, appreciating the difference between light and darkness, shelter themselves in dark places, survive. Hence the tendency of natural selection is to prevent the decrease of the eyes beyond that point at which they can distinguish between light and darkness. Thus the apparent anomaly is explained.
- Essays upon Heredity, p. 87.