To put the matter more generally, it is not enough to show that some modification in the parent results in some modification in the child. For the purposes of this argument there must be a similar modification.
(8) Finally, data are frequently presented, which cover only two generations—parent and child. Indeed, almost all the data alleged to show the inheritance of acquired characteristics are of this kind. They are of little or no value as evidence. Cases covering a number of generations, where a cumulative change was visible, would be of weight, but on the rare occasions when they are forthcoming, they can be explained in some other way more satisfactorily than by an appeal to the theory of Lamarck.[13]
If the evidence currently offered to support a belief in the inheritance of acquired characters is tested by the application of these "misunderstandings," it will at once be found that most of it disappears; that it can be thrown out of court without further formality. The Lamarckian doctrine is now held mainly by persons who have either lacked training in the evaluation of evidence, or have never examined critically the assumptions on which they proceed. Medical men and breeders of plants or animals are to a large extent believers in Lamarckism, but the evidence (if any) on which they rely is always susceptible of explanation in a more reasonable way. It must not be forgotten that some of the ablest intellects in the world have been assidously engaged in getting at the truth in the case, during the last half-century; and it is certainly worthy of consideration that not in a single case has the transmission of an acquired body character ever been proved beyond dispute. Those who still hold a belief in it (and it is fair to say that some men of real ability are among that number) too often do so, it is to be feared, because it is necessary for the support of some theoretical doctrine which they have formulated. Certainly there are few men who can say that they have carefully examined the evidence in the case, and accept Lamarckism because the evidence forces them to do so. It will be interesting to review the various classes of alleged evidence, though we can cite only a few cases from the great number available (most of them, however, dealing with plants or lower animals).
Nearly all the evidence adduced can be put in one of these four classes:
(1) Mutilations.
(2) Diseases.
(3) Results of use or disuse.
(4) Physico-chemical effects of environment.
The case in regard to mutilations is particularly clear cut and leaves little room for doubt. The noses and ears of oriental women have been pierced for generations without number, yet girls are still born with these parts entire. Circumcision offers another test case. The evidence of laboratory experiments (amputation of tails) shows no inheritance. It may be said without hesitation that mutilations are not heritable, no matter how many generations undergo them.
(2) The transmissibility of acquired diseases is a question involved in more of a haze of ignorance and loose thinking. It is particularly frequent to see cases of uterine infection offered as cases of the inheritance of acquired characters. To use the word "heredity" in such a case is unjustified. Uterine infection has no bearing whatever on the question.
Taking an historical view, it seems fairly evident that if diseases were really inherited, the race would have been extinct long ago. Of course there are constitutional defects or abnormalities that are in the germ-plasm and are heritable: such is the peculiar inability of the blood to coagulate, which marks "bleeders" (sufferers from hemophilia, a highly hereditary disease). And in many cases it is difficult to distinguish between a real germinal condition of this sort, and an acquired disease.
The inheritance of an acquired disease is not only inconceivable, in the light of what is known about the germ-plasm, but there is no evidence to support it. While there is most decidedly such a thing as the inheritance of a tendency to or lack of resistance to a disease, it is not the result of incidence of the disease on the parent. It is possible to inherit a tendency to headaches or to chronic alcoholism; and it is possible to inherit a lack of resistance to common diseases such as malaria, small-pox or measles; but actually to inherit a zymotic disease as an inherent genetic trait, is impossible,—is, in fact, a contradiction of terms.
(3) When we come to the effects of use and disuse, we reach a much debated ground, and one complicated by the injection of a great deal of biological theorizing, as well as the presence of the usual large amount of faulty observation and inference.