216 



The Journal of Heredity 



one-fourth of the j^rogeny of such 

 matings show feeblemindedness. There- 

 fore, at least 100,000 couples of this 

 type were rei)roducing during this 

 generation. This would presuppose the 

 survival of four children per couj^le 

 long enough to have their mental status 

 determined, an assumption that would 

 probably require a total reproductivity 

 of seven children ])er married ]jair. 

 Amoog the children from these matings 

 would be some 200,000 carriers of de- 

 fective germ-cells, but we will omit 

 them from our considerations. The 

 important point is that out of 20,000,000 

 pairs of married persons, if we treat the 

 problem as static, 100,000 were trans- 

 mitting feeblemindedness. What then 

 is the number of such jjersons in the 

 ponulation ' 



Let us state the question in another 

 way. A certain number of ])ersons 

 out of a population of 40,000,000 of a 

 marriageable age have defective germ- 

 cells. If two of them marry, one-quarter 

 of their children will be feebleminded. 

 If 100,000 of such marriages did occur, 

 what is the ratio of carriers of feeble- 

 mindedness to normals in the general 

 ])o]julation? The correct answer will 

 de]jend of course upon how much selec- 

 tive mating takes place. There is un- 

 questionably a general tendency for 

 carriers of feeblemindedness to be 

 brought together and a marriage to 

 result. But this cannot be taken into 

 account very accurately and had best 

 be left out of our calculations. 



Pairing among carriers of feeble- 

 mindedness has occvirrcd in the ratio 

 of 1 to 200 marriages; then, if no selec- 

 tive mating has taken place, carriers 

 of feeblemindedness must occur in the 

 general po])ulation in the ratio of I 

 to 14. 



One-fourteenth is approximateh' the 

 square root of 1, 200. If 1, 14 of the 

 poi)ulation carry feeblemindedness and 

 13/14 are normal, then the pr()bal)ility 

 of normal mating with normal is 13/ 14X 

 13/14=169/1%, the probability of nor- 

 mal mating with carriers of feeblemind- 

 edness is 1/14X13/14-^13/14X1/14 

 = 26/196, and the probability of two 

 carriers of feeblemindedness mating is 

 1/14X1/14=1 196. 



Possibly this figure is somewhat too 

 high for the single trait feebleminded- 

 ness. We have not corrected for 

 changes in the po]nilation during the 

 length of the period considered or for 

 selective mating. But, to balance this 

 we have used a low estimate of the 

 number of feebleminded, a high esti- 

 mate of the number of defecti\'es pro- 

 duced by parents of which at least one 

 exhibited defects, and a high birth-rate 

 in families of those transmitting the 

 defect. Further, no mention has been 

 made of epilepsy and of certain types of 

 insanity, which are inherited in the 

 same way, and to which the same line of 

 reasoning applies. In view of these 

 facts it is probable that the conclusion 

 that 1 ])erson out of every 14 carries 

 the basis of serious mental defective- 

 ness in one-half of his or her reproduc- 

 tive cells understates rather than over- 

 states the facts. 



The problem of cutting off defective 

 germ-plasm, therefore, is not the com- 

 paratively simple one of preventing the 

 multi]3lication of those so affected. 

 This task, though sufficiently difficult in 

 practice, is possible: the way has been 

 pointed out; something has been ac- 

 complished. It is rather the almost 

 hojjeless task of reducing the birth- 

 rate among transmitters of serious 

 defects. 



NEED FOR RESE.\KCH 



A stupendous task necessitates pro- 

 digious efforts. Already there is a 

 tremendous selective birth-rate in favor 

 of lesser civic worth, and it is extremely 

 doubtful whether, under our j^resent 

 economic system, much can be accom- 

 ])lished by recommending early marri- 

 ages and large families among those 

 whose accom])lishments have pnn-ed 

 their social value. Whether family 

 limitation among those carrying defec- 

 tive germ-i)lasms can be effected must 

 be decided in the future. It will be a 

 distruit future if a stujjid government 

 jXTsists in refusing to countenance 

 rational ])arenthood among those least 

 fitted to rei)roduce the race, the while 

 shutting one eye and winking the other 

 at what has become a national i)ractice 

 among those best fitted to build a 



