The question of inherited immunity to diseases, as the result of vaccination or actual illness from them, has appeared in the controversy in a number of forms, and is a point of much importance. It is not yet clear, partly because the doctors disagree as to what immunity is. But there is no adequate evidence that an immunity to anything can be created and transmitted through the germ-plasm to succeeding generations.
In short, no matter what evidence we examine, we must conclude that inheritance of acquired bodily characters is not a subject that need be reckoned with, in applied eugenics.
On the other hand, there is a possible indirect influence of modifications, which may have real importance in man. If the individual is modified in a certain way, in a number of generations, even though such a modification is not transmitted to his descendants, yet its continued existence may make possible, the survival of some germinal variation bearing in the same direction, which without the protecting influence of the pre-existing modification, would have been swamped or destroyed.
Finally, it should be borne in mind that even if physical and mental characters acquired during a man's lifetime are not transmitted, yet there is a sort of transmission of acquired characters which has been of immense importance to the evolution of the race. This is the so-called "inheritance" of the environment; the passing on from one generation to the next of the achievements of the race, its accumulated social experience; its civilization, in short. It is doubtful whether any useful end is gained by speaking of this continuance of the environment as "heredity;" it certainly tends to confuse many people who are not used to thinking in biological terms. Tradition is the preferable term.
There is much to be said in favor of E. B. Poulton's definition,—"Civilization in general is the sum of those contrivances which enable human beings to advance independently of heredity." Whatever wisdom, material gain, or language is acquired by one generation may be passed on to the next. As far as the environment is concerned, one generation stands on the shoulders of its predecessor. It might simplify the task of eugenics if the same could be said of biological heredity. But it can not. Each generation must "start from scratch."
In August Weismann's words, the development of a function in offspring begins at the point where it began in his parents, not at the point where it ended in them. Biological improvement of the race (and such improvement greatly fosters all other kinds) must be made through a selective birth-rate. There is no short-cut by way of euthenics, merely.
We must now consider whether there is any direct way of impairing good heredity. It is currently believed that there are certain substances, popularly known as "racial-poisons," which are capable of affecting the germ-plasm adversely and permanently in spite of its isolation and protection. For example, the literature of alcoholism, and much of the literature of eugenics, abounds with statements to the effect that alcohol originates degeneracy in the human race.
The proof or disproof of this proposition must depend in the last analysis on direct observation and carefully controlled experiments. As the latter cannot be made feasibly on man, a number of students have taken up the problem by using small animals which are easily handled in laboratories. Many of these experiments are so imperfect in method that, when carefully examined, they are found to possess little or no value as evidence on the point here discussed.
Hodge, Mairet and Combemale, for example, have published data which convinced them that the germ-plasm of dogs was injured by the administration of alcohol. The test was the quality of offspring directly produced by the intoxicated animals under experiment. But the number of dogs used was too small to be conclusive, and there was no "control": hence these experiments carry little weight.