30 GENETICS AND EUGENICS 



the practical outcome is very similar; an individual once in- 

 fected with syphilis is racially condemned; his seed is as 

 truly bad as if the syphilis germ were an essential part of the 

 germinal substance. 



(b) The intimate relationship of parent to child may give 

 unusual opportunities for post-natal infection, as in the case 

 of tuberculosis. Thus the children of tuberculous parents 

 are more liable to infection with tuberculosis, other things 

 being equal, than the children of non-tuberculous parents. 

 But we are not justified for that reason in speaking of tuber- 

 culosis as hereditary. It is probably in all cases acquired by 

 the patient, individually, and not inherited. Whether some 

 individuals are more susceptible than others is a wholly 

 different question. Susceptibility may well be inherited. 



(c) Just as a disease-producing organism may be received 

 into the egg or the embryo while it is still within the body of 

 the mother, so chemical substances in the mother's blood may 

 enter the egg or embryo and affect its subsequent character. 

 Thus it has been shown that in guinea-pigs immunity ac- 

 quired by the mother (which is known to be due to the 

 presence of specific substances in the blood) may be trans- 

 mitted to her offspring, though the father has no such in- 

 fluence, the reason being that the sperm-cell is too small to 

 carry an effective quantity of antitoxin, i. e., of immunity 

 producing substance. In such cases as I have just mentioned 

 of transmitted immunity, the immunity does not last beyond 

 a single generation. It has not become hereditary, it has 

 simply been passively received by the embryo. 



On the whole, we must conclude that disease transmission 

 furnishes no evidence in favor of the transmission of acquired 

 characters. The most debatable case is that of acquired 

 disease transmitted in the germ-cell. For practical purposes 

 this is heredity. For truly hereditary characters are often as 

 detachable and separate from the germ-cell as foreign bodies, 

 as we shall see when we come to study Mendelian inheritance. 



3. Induced epilepsy. A famous case cited in all discussions 

 of this subject is the case of Brown-Sequard's guinea-pigs. 



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