We might be inclined to regard this gradual disappearance and ultimate elimination of a disused organ as a direct gain, on the ground that the economy of material and space thus effected may be of decided advantage to the individual animal and thereby also for the maintenance of the species, and that those animals would have an advantage in the struggle for existence in which the superfluous organ was reduced to the smallest expression. But that would be far from supplying us with a sufficient explanation of the phenomenon; the individual variations in the size of an organ which is in process of degenerating are, even in extreme cases, far too slight to have any selection-value, and I cannot call to mind a single case in which the contrary could be assumed with any degree of probability. What advantage can a newt or a crustacean living in darkness derive from the fact that its eye is smaller and more degenerate by one degree of variation than those of its co-partners in the struggle for existence? Or, to use Herbert Spencer's striking illustration, how could the balance between life and death, in the case of a colossus like the Greenland whale, be turned one way or another by the difference of a few inches in the length of the hind-leg, as compared with his fellows, in whom the reduction of the hind-limb may not have gone quite so far? Such a slight economy of material is as nothing compared with the thousands of hundredweights the animal weighs. As long as the limbs protrude beyond the surface of the trunk they may prove an obstacle to rapid swimming, although that could hardly make much difference, but as soon as the phyletic evolution had proceeded so far that they were reduced to the extent of sinking beneath the surface, they would no longer be a hindrance in swimming, and their further reduction to their modern state of great degeneration and absolute concealment within the flesh of the animal cannot be referred even to negative selection. Years ago I endeavoured to explain the degeneration of disused parts in terms of a process which I called Panmixia. Natural selection not only effects adaptations, it also maintains the organ at the pitch of perfection it has reached by a continual elimination of those individuals in which the organ in question is less perfect. The longer this conservative process of selection continues, the greater must be the constancy of the organ produced by it, and deviations from the perfect organ will be of less and less frequent occurrence as time goes on.