One of the key aspects of science controversy – that is people disagree with you and say so. Liu-Ochman flagellum evolution paper paper is a good example. Disagreements are a crucial aspect of how science progresses. In this example Darwin is defending one of his statements about barnacles (the discussion is taking place in the pages of Nature):
I BEG permission to make a few remarks bearing on Prof. Wyville Thomson’s interesting account of the rudimentary males of Scalpellum regium, in your number of August 28th. Since I described in 1851, the males and complemental males of certain cirripedes, I have been most anxious that some competent naturalist should re-examine them; more especially as a German, without apparently having taken the trouble to look at any specimens, has spoken of my description as a fantastic dream. That the males of an animal should be attached to the female, should be very much smaller than, and differ greatly in structure from her, is nothing new or strange.
In the course of his defense Darwin discusses the rudimentary nature of some male barnacles and goes on to say:
It is known from the researches of Quetelet on the height of man, that the number of individuals who exceed the average height by a given quantity is the same as the number of those who are shorter than the average by the same quantity; so that men may be grouped symmetrically about the average with reference to their height. I may add, to make this clearer, that there exists the same number of men between three and four inches above the average height, as there are below it. So it is with the circumference of their chests; and we may presume that this is the usual law of variation in all the parts of every species under ordinary conditions of life. That almost every part of the body is capable of independent variation we have good reason to believe, for it is this which gives rise to the individual differences characteristic of all species. Now it does not seem improbable that with a species under unfavourable conditions, when, during many generations, or in certain areas, it is pressed for food and exists in scanty numbers, that all or most of its parts should tend to vary in a greater number of individuals towards diminution than towards increment of size; so that the grouping would be no longer symmetrical with reference to the average size of any organ under consideration. In this case the individuals which were born with parts diminished in size and efficiency, on which the welfare of the species depended, would be eliminated; those individuals alone surviving in the long run which possessed such parts of the proper size. But the survival of none would be affected by the greater or less diminution of parts already reduced in size and functionally useless. We have assumed that under the above stated unfavourable conditions a larger number of individuals are born with any particular part or organ diminished in size, than are born with it increased to the same relative degree; and as these individuals, having their already reduced and useless parts still more diminished by variation under poor conditions, would not be eliminated, they would intercross with the many individuals having the part of nearly average size, and with the few having it of increased size. The result of such intercrossing would be, in the course of time, the steady diminution and ultimate disappearance of all such useless parts. No doubt the process would take place with excessive slowness; but this result agrees perfectly with what we see in nature; for the number of forms possessing the merest traces of various organs is immense. I repeat that I have ventured to make these hypothetical remarks solely for the sake of calling attention to this subject.
In a later issue of Nature he has his son – George – write in and clarify a few issues:
Supposing then that a race of cattle becomes exposed to unfavourable conditions, my father’s hypothesis is that, whilst the larger proportion of the cattle have their horns developed in the same degree as though they had enjoyed favourable conditions, the remainder have their horns somewhat stunted. Now, if we had made a record of the length of horn in the same species under favourable conditions, we should, as in the case of the heights of men, have a central cluster, with a symmetrical distribution of the pins above and below the cluster. According to the hypothesis, the effect of the poor conditions may be represented by the removal of a certain proportion of the pins, taken at hazard, to places lower down, whilst the rest remain in statû quo. By this process the central cluster will be slightly displaced downwards, since its upper edge will be made slightly less dense, whilst its lower edge will become denser; and further, the density of distribution will diminish more rapidly above than below the new central cluster.
Now, if horns are useful organs, the cattle with shorter horns will be partially weeded out by natural selection, and will leave fewer offspring; and after many generations of the new conditions, the symmetry of distribution of the pins will be restored by the weeding out of some of those below the cluster, the central cluster itself remaining undisturbed.
If, on the other hand, horns are useless organs, the cattle with stunted horns have as good a chance of leaving offspring (who will inherit their peculiarity) as their long-horned brothers. Thus, after many generations under the poor conditions, with continual intercrossing of all the members, the symmetry of distribution will be again restored, but it will have come about through the general removal of all the pins downwards, and this will of course have shifted the central cluster.
If, then, the poor conditions produce a continuous tendency to a stunting of the nature above described, there will be two operations going on side by side–the one ever destroying the symmetry of distribution, and the other ever restoring it through the shifting of the cluster downwards.
Thus, supposing the hypothesis to be supported by facts (and my father intends to put this to the test of experiment next summer), there is a tendency for useless organs to diminish and finally disappear, besides those arising from disuse and the economy of nutrition.
I have not been able to track this experiment down yet (I’m still working on it), but this displays, yet again, the fact that Darwin was indeed working in the best scientific tradition and experimentally testing his hypotheses. Interestingly enough, in later issues of Nature George Romanes picks up on the issue of variation of organs coming to similar conclusions – as Darwin mentions in The Variation of Animals and Plants Under Domestication:
Mr. Romanes has, I think, thrown much light on this difficult problem. His view, as far as it can be given in a few words, is as follows: all parts are somewhat variable and fluctuate in size round an average point. Now, when a part has already begun from any cause to decrease, it is very improbable that the variations should be as great in the direction of increase as of diminution; for the previous reduction shows that circumstances have not been favourable for its development; whilst there is nothing to check variations in the opposite direction. If this be so, the long continued crossing of many individuals furnished with an organ which fluctuates in a greater degree towards decrease than towards increase, will slowly but steadily lead to its diminution. With respect to the complete and absolute abortion of a part, a distinct principle, which will be discussed in the chapter on pangenesis, probably comes into action.
Filed under: The Experimental Darwin