MAN AND MANKIND 



universal. Diclil and Verschucr compared susceptibility in one-egg 

 and in two-egg (or fraternal) twins, one or both of which liad 

 tuberculosis (Table 29). 



They found that the identical twins do not always show the same 

 susceptibilities. This is due to their environmental differences. But 

 they show much more similar susceptibilities than the fraternals. 

 This is due to their genetic identity. The difference between identical 

 and fraternal twins is thus equally significant in showing that 

 environmental differences on the one hand, and genetic differences 

 on the other, affect susceptibility. This principle has been carried 

 further by Kallmami and Reisner in showing the gradual reduction 



TABLE 30 



CONCORDANCE IN INCIDENCE OF TUMOURS, TIME OF 

 ONSET, AND TYPE OF TUMOUR IN ONE-EGG AND 

 TWO-EGG TWINS IN MAN (MACKLIN, 1940) 



in correlation between individual susceptibilities to tuberculosis as 

 genetic relationship decreases. Thus where one member of a pair 

 is affected the other member is also affected in the proportions of 

 cases shown in Table 31. 



The difference between the proportion of full siblings and of 

 parents and offspring, where the same degree of genetic relationship 

 is involved, is due to the fact that different generations obviously 

 must differ in environment more than the same generation. The 

 gradation, therefore, again neatly reflects the reaction of differences 

 in both heredity and environment. 



The second special development of heredity in man is in the study 

 of blood groups whose physiological and evolutionary interest w^e 

 have already examined (Fig. 37). This study was called into being by 

 the needs of transfusion, since bloods containing A and B cannot be 

 mixed. It developed a secondary interest as a means of proving 



350 



