The adverse instances are very clear indeed. Chinese girls are never born with abnormally small feet. Jews are not born circumcised. Among tribes where tattooing is practised, no traces of this embellishment are ever found to be inherited. If it is a physiological law that the disuse of an organ not only atrophies it in the individual but (by inheritance of the atrophy) eliminates it from the species, there is no apparent reason why this law should not operate in cases where the organ is artificially removed. Yet it rarely or never seems to do so. Experiments upon animals, such as breeding for many generations from mice whose tails have been cut off, have never resulted in producing a clear case of inherited mutilation. A strong presumption is therefore raised that the effects apparently due to use and disuse under natural conditions (as in the eyeless fishes of the Kentucky caves) must be set down to some other cause. The queens in colonies of ants and bees have never exercised the functions of workers for thousands of centuries, yet they transmit these functions unimpaired.
There is, indeed, a case often referred to in this connexion which must be here mentioned. Dr. Brown-Séquard found that by injuring or compressing the sciatic nerve in guinea-pigs epilepsy was produced, and that the descendants of animals so injured had a marked tendency to epileptic fits. This is undoubtedly a very significant and important fact in biology, but it gives no support to the Lamarckian theory. What is inherited by the guinea-pigs is not the injury to the nerve but the pathological condition resulting therefrom. It remains to be discovered how, precisely, this takes place, and the experiment may end in illuminating a very obscure region in physiology, but on Lamarckism it has no bearing at all. A better case is that of atrophy of a toe, which is said to have been inherited in consequence of its original production by severance of the sciatic nerve, but, again, what is inherited is not an actual injury but an effect of it. It is clear, however, that bodily conditions of a large and comprehensive kind produced naturally or artificially in an individual may have an effect on the reproductive cells, especially when the nervous system is affected.
Coming to the observation of what happens under natural conditions, we are struck at the outset by the fact that the inheritance of acquired characteristics, if it works at all, must work under some system of salutary control and not as a blind physiological law. For if each generation starts with some measure at least of what the former generation had acquired, and adds to it by its own activity, then all acquired characteristics would ere long attain a monstrous development, and the species would perish under them. But nothing of the kind is observed to happen. The continual use of the muscles in the labouring classes has not made men stronger than they were thousands of generations ago. The habit of handling the spade and hoe has never produced a peasant child born with callosities on its hands. The horn of the rhinoceros, which on Lamarckian principles we must regard as developed by the gradual increase of a callosity formed by grubbing for roots, does not grow beyond a certain size, however the species may go on grubbing. The Lamarckian law, then, if it has any real effect at all, can only express half the truth about the action of heredity on acquired characteristics. As the column of water in a fountain hovers about a certain height, so the action of heredity in the accumulation of the effects produced by the use of organs seems to have a limit beyond which it cannot pass. May it not be that heredity is really as false an expression for the phenomenon as the popular superstition about ‘water seeking its own level’ is for the upspringing of a fountain?
The cases of co-adaptation, where one organ appears to be developed by use and others by the use of that, as in the case of the Irish elk referred to above, are met by instances just as striking where the elements of modification by use cannot come into play. Weismann mentions the case of the ingenious brush arrangement on the anterior legs of the bee, which the insect uses for cleansing its antennæ. Two adaptations are here developed—a little semicircular notch in the leg, set with small bristles, and a movable projection or flap used for pressing the antenna into the notch as it is drawn through. The bee, no doubt, would naturally try to clean its antennæ with its fore-legs, but how could this process develop the special arrangements referred to in the hard or scaly covering of its limbs? It is not until the shell of the insect has grown quite hard and incapable of further vital changes that the arrangement comes into use. Again, the stridulating noise produced by the legs of the grasshopper is due to serrations occurring on different joints of the limb. Serrations on one joint would in no way tend to develop them on the other, but rather the contrary, yet there they are, in harmonious co-operation. If Nature can obtain these effects, as she does in numberless instances, without the aid of Lamarck’s principle, we cannot help asking whether that principle is ever operative at all.
The three instances which we shall next consider seem to offer very serious obstacles to the Lamarckian theory.
A modification of structure caused by the special use of a certain organ takes place in probably over 90 per cent of the whole human race, male and female. The records of art, of language, and the evidence of actual remains, tend to show that the habitude in question, with the attendant modification, goes back to very ancient, even perhaps to palæolithic times.[60] I refer to the preferential use of the right hand and the enlargement of structure thus brought about in the right hand and arm. Every right-handed adult man and woman shows this enlargement of bony and muscular structure. The origin of the habitude does not concern us here. Let us suppose it due, as Dr. D. J. Cunningham suggests, to “a transmitted functional pre-eminence of the left brain,”[61] which is larger than the right, and which governs the movements of the right side of the body. However this may be, it is clear that if bodily characteristics acquired by exercise are transmissible by inheritance the new-born child of right-handed ancestry ought to show some appreciable preponderance in weight and size of the right over the left limb. There could hardly be a more crucial test of the validity of the Lamarckian principle. What do the investigations of the dissecting-room reveal? I shall quote the two most recent authorities who have studied this interesting question. Dr. Cunningham, in the lecture already referred to, writes:—
“Although the matter has not been investigated so fully as to place the question outside the region of dispute, the evidence at our disposal distinctly favours the view that at birth the two upper limbs start upon their individual duties equally endowed in so far as strength of muscle and size of bones are concerned. Both in mass and weight the two limbs are to all intents and purposes similar at birth, and the preponderance in bulk and strength which later on distinguishes the right arm is acquired during life, and is caused by the greater amount of work it is called upon to perform.”[62]
Dr. T. G. Moorhead, Chief Demonstrator in Anatomy in Trinity College, Dublin, after giving the results of the researches of various other inquirers, writes:—
“From this mass of conflicting evidence I am forced to the conclusion that no real differences exist.... After weighing as a whole the limbs of eight foetuses I was unable to detect any constant difference.”[63]