The Evolution Theory, Vol. 2 of 2

by August Weismann

On what does disappearance after disuse depend, if not on the Lamarckian principle?—Panmixia—Romanes—Fluctuations in the determinant-system of the germ-plasm due to unequal nutrition—Persistence of germinal variations in a definite direction—The disappearance of non-functioning parts—Preponderance of minus germinal variations—Law of the retrogression of useless parts—Variation in an upward direction—Artificial selection—Influence of the multiplicity of ids and of sexual reproduction—Personal selection depends on the removal of certain id-variants—Range of influence of germinal selection—Self-regulation of the germ-plasm, which is striving towards stability—Ascending variation-tendencies may persist to excess—Origin of secondary sexual characters—Significance of purely morphological characters—The markings of butterflies.

Now that we have recognized that the assumption of a transmission of functional modifications is not justifiable, let us discuss some of the many phenomena to explain which many people believe the Lamarckian principle to be indispensable, and let us inquire whether we are in a position to give any other explanation of these. How has it come about that the effects of use and disuse appear to be inherited? Can we find a sufficient explanation in the principle of selection, and in the natural selection of Darwin and Wallace?

The answer to these two questions will be most quickly found if we begin by seeking for an explanation of the disappearance of a part when it ceases to be exercised.

That this cannot lie in the Lamarckian principle we have already learnt from the fact that passively functioning parts, such as superfluous wing-veins, also disappear, and that the loss of the wings and degeneration of the ovaries has taken place in worker ants, which can transmit nothing because they do not reproduce.

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.

Now if this conservative action of natural selection secures the maintenance of the parts and organs of a species at their maximum of perfection, it follows that these will fall below this maximum as soon as the selection ceases to operate. And it does cease as soon as an organ ceases to be of use to its species, like the eye to the species of crustacean which descends into the dark depths of our lakes, or to the abyssal zones of the ocean, or into a subterranean cave-system. In this case all selection of individuals ceases as far as the eye is concerned; it has no importance in deciding survival in the struggle for existence, because no individual is at a disadvantage through its inferior eyes, for instance, by being in any way hindered in procuring its food. Those with inferior organs of vision will, ceteris paribus, produce as good offspring as those with better eyes, and the consequence of this must be that there will be a general deterioration of eyes, because the bad ones can be transmitted as well as the good, and thus the selection of good eyes is made impossible.

The mixture thus arising may be compared to a fine wine to which a litre of vinegar has been added; the whole cask is ruined because the vinegar mingles with every drop of the wine. As deviations from the normal are always occurring in every part of every species, and among them some that lessen the value of the organ, rarely perhaps at first, but after a time in every generation, a sinking of the organ from the highest point of possible perfection becomes inevitable as soon as the organ becomes superfluous. The functional uselessness of the organ must go on increasing the longer it is disused, as will readily be admitted if it be remembered that only the most perfect adaptation of all the separate parts of an organ can maintain its functional capacity, that all the parts of an organ are subject to variation, and that every deviation from the optimum implies a further deterioration of the whole. An eye, for instance, can no longer vary in the direction of 'better' if it has already reached the highest possible point of perfection; every further variation must deteriorate it.

Romanes gave expression to this idea, that the cessation of natural selection alone must cause the degeneration of a part, a decade before I did, but neither he nor the scientific world of his time attached great importance to it, and it was forgotten again. This was intelligible enough, for, at that time, the validity of the Lamarckian principle had not been called in question, and therefore the need for some other principle to explain the disappearance of disused parts had not begun to be felt.

I found myself in quite a different position. As my doubts regarding the Lamarckian principle grew greater and greater, I was obliged to seek for some other factor in modification, which should be sufficient to effect the degeneration of a disused part, and for a time I thought I had found this in panmixia, that is, in the mingling of all together, well and less well equipped alike. This factor does certainly operate, but the more I thought over it the clearer it became to me that there must be some other factor at work as well, for while panmixia might explain the deterioration of an organ, it could not explain its decrease in size, its gradual wearing away, and ultimate total disappearance. Yet this is the path followed, slowly indeed, but quite surely, by all organs which have become useless. If panmixia alone guided the deterioration of the organ, and it was thus only chance variations which were inherited through panmixia and gradually diffused over the whole species, how could it come about that all the variations were in the direction of smaller size? Yet this is obviously the case. Why should no variations in the direction of larger size occur? And if this were so, why should a useless organ not be maintained at its original size, if it be admitted that an increase in size would be prevented by natural selection? But this never occurs, and diminution in size is so absolutely the rule that the idea of a 'vestigial or rudimentary' organ suggests a 'small organ' almost more than an 'imperfect' one.