[147] Of course it must be observed that degeneration of complexity involves also degeneration of size, so that a more correct statement of the case would be--Why, under the cessation of selection, does an organ of extreme complexity degenerate much more rapidly than one of much less complexity? For example, under domestication the brains of rabbits and ducks appear to have been reduced in some cases by as much as 50 per cent. (Darwin, and Sir J. Crichton Browne.) But if it is possible to attribute this effect--or part of it--to an artificial selection of stupid animals, I give in the text an example occurring under nature. Many other cases, however, might be given to show the general rule, that under cessation of selection complexity of structure degenerates more rapidly--and also more thoroughly--than size of it. This, of course, is what Mr. Galton and I should expect, seeing that the more complex a structure the greater are the number of points for deterioration to invade when the structure is no longer "protected by selection." (On the other hand, of course, this fact is opposed to the view that degeneration of useless structures below the "birth-mean" of the first generations, is exclusively due to the reversal of selection; for economy of growth, deleterious effect of weight, and so forth, ought to affect size of structure _much more_ than complexity of it.) But I choose the above case, partly because Professor Lloyd Morgan has himself alluded to "the eyes of crustacea," and partly because Professor Ray Lankester has maintained that the loss of these eyes in dark caves is due to the reversal of selection, as distinguished from the cessation of it. In view of the above parenthesis it will be seen that the point is not of much importance in the present connexion; but it appears to me that cessation of selection must here have had at least the larger share in the process of atrophy. For while the economy of nutrition ought to have removed the relatively large _foot-stalks_ as rapidly as the _eyes_, I cannot see that there is any advantage, other than the economy of nutrition, to be gained by the rapid loss of hard-coated _eyes_, even though they have ceased to be of use.