In a similar manner Rennert[87] has shown that lead-poisoning occurring in the mother is apt to produce macrocephalism with frequent idiocy in the child.
This brings the observer face to face with the problem of morbid heredity. It may at once be admitted that, as has been claimed by a large number of observers, morbid heredity, especially in its graver forms, is much less frequent than at first would be expected. J. P. Gray, of Utica, New York, went so far as to claim that disease is never transmitted, but this is contradicted by his own hospital reports, which, to the day of his death, contained a table headed, “Statistics of hereditary transmission of the disease.” It is true, however, that the descendants of a victim of morbidity or abnormality do not always exhibit the morbidity or abnormality of the ancestor. In some cases all apparent morbidity or abnormality is wanting. In other cases slighter abnormalities are to be detected. Here the observer is brought face to face with the operation of two general principles which are interdependent: the transmutation of heredity and atavism, or “throwing back” as the cattle breeders call it. Tennyson voices the general erroneous opinion of the always evil effects of atavism in his “Locksley Hall Sixty Years After”:—
“Evolution ever climbing after some ideal good
And Reversion ever dragging Evolution in the mud.”
As Kiernan has shown, atavism at times tends to preserve the type, and offsets the influence of degeneracy. This element of atavism underlies not merely the production of the sound scions of degenerate stock, but also those in whom the degeneracy affects the earlier and not the later acquirements of the race. The contrast of the moral imbecile who is unable to acquire an idea of right, or the idiot of the lowest grade who can hardly be taught to keep himself clean, with the otherwise sound, sane, able victim of hereditary gout, is very great. Yet all the links of the chain connecting, in the same family, these contrasted types, can often be found. The law laid down as to absolute extinction through degeneracy by Morel and others can only be regarded as absolutely true when applied to a given type rather than the race as a whole. Indeed, environment may play a part in preserving a degenerate who would otherwise die out. Thus in societies at a certain stage of culture imbeciles, paupers, lunatics, and congenital criminals live at large and even propagate through legal marriage. The seemingly enormous increase of the defective class which occurs in frontier communities when the classes begin to be placed in public institutions is an excellent illustration of this.
Manifestations of morbid heredity result not in inheritance of the whole defect but in disturbance of relations of structure and hence of function, producing, as Kiernan remarks, a constitutional deficiency which takes the line of least resistance. The extent and direction of this line of least resistance depend on the amount of healthy atavism which separate organs and structures of the body preserve. The line of least resistance sometimes taken is excellently illustrated in the occurrence of stigmata already pointed out in the case of epileptic guinea-pigs.
This unilateral predisposition (which, as Kiernan has shown, is due usually to heredity or intra-uterine causes) may be artificially produced. Kasparek[88] cut one sciatic nerve of a guinea-pig and (after all inflammatory symptoms had subsided) injected the opposite ear with cultures of pus microbes. He killed the animal after some days. The sound side was found free from suppuration, but immense abscesses existed on the side of the cut nerve. Such local predisposition was pointed out by Merrill[89] over forty years ago. This condition occurs, as Féré has shown,[90] in many systemic and infectious diseases which presents a localisation due to heredity, or determined by anterior morbid state of the nervous symptom. Sometimes these manifestations are limited to the side free from nervous trouble.
As a rule they attack the side which is the predominant seat of nervous symptoms. Féré points out that in chromatic iris asymmetry the iris (coloured part of the eye) is most coloured on the side most affected by arrest of development. Localisation of nervous trouble occurs on the side most affected by hare-lip. Heuse has observed the co-existence on the same side of congenital cataract and of deformities of the skull and chest.
Hernia is often an expression of hereditary defect (Le Double) taking the unilateral line of least resistance. Testicle inflammations of microbic origin (venereal or otherwise) occur as a rule on the side where hernia is located in the groin. In one-sided malformations of ovary or testicle (decreased or increased in size, or changed in shape or location) microbe inflammation almost always occurs at the seat of the anomaly. This principle is illustrated in the experiments of Dupuy.[91] Here, while as a rule, the scions of guinea-pigs (rendered epileptic by section of the sciatic nerve) were epileptics and had deficient toes, still in some epilepsy resulted without the toe anomaly, while still more rarely the toe anomaly was present without the epilepsy.
The same principle is shown by certain observations of Charin and Gley,[92] who for five years conducted experiments calculated to throw light on the influence on the offspring of parental reception of virus. Either both male and female have been inoculated with the bacillus of blue pus or its toxins, or but one animal has been inoculated. The results have not been uniform. Most frequently there ensues sterility, abortion, or birth of progeny that die immediately. In rare instances the offspring survive; more rarely still are they healthy. Certain rabbits (born of these undeveloped animals) were provided with enormous epiphyses (ends of bones), the shafts of the bones being shortened. Two rabbits were born of a couple of which the male alone received inoculations of sterilised culture. Five rabbits were born of these two, of which two were normal, and a third (whose ears were rudimentary) died in a few days. In the remaining two the ears comprised only fragments with jagged upper edges. The tails were but two centimetres long. The external orifice of the vagina (one rabbit was a male and the other a female) was oblique. One of the limbs (the hind in the male and the fore in the female) was much shorter than its fellow, the difference being four centimetres. The shortened limb ended in a kind of stump, there being no foot or toes.
These experiments illustrate the transformation of heredity, that is the manifestations which show the line of least resistance that the morbid heredity has taken. As Moreau (de Tours) remarks,[93] “An incorrect conception of the law of heredity looks for identical phenomena in each succeeding generation. Some have refused to admit that mental faculties were subject to heredity, because the mental characters of the descendants were not precisely those of the progenitors. Each generation must copy the preceding. Father and son must present the spectacle of one being, having two births, and each time leading the same life, under the same conditions. But it is not in the heredity of functions, or of organic or intellectual facts that the application of the law of heredity must be sought, but at the very fountain-head of the organism, in its inmost constitution. A family whose head is insane or epileptic does not of necessity consist of lunatics or epileptics, but the children may be idiotic, paralytic, or scrofulous. What the parents transmit to the children is not insanity, but a vicious constitution which will manifest itself under various forms in epilepsy, hysteria, scrofula, rickets, &c. This is what is to be understood by hereditary transmission.”