412 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



ing. It is the case of twin brothers called " univitellins." There are 

 twins called " bivitellins,'- who owe their origin to the development 

 of two eggs side by side ; these do not resemble each other more than 

 two ordinary brothers. But there are also twins called " univitel- 

 lins," who owe their origin to the fact that a single egg is primitively 

 cut in two and each half has developed a complete being. Then the 

 identity is complete, and there is no more striking demonstration of 

 the power of heredity than to see these two beings, who are some- 

 times brought up under different conditions, who choose different 

 careers, and who, nevertheless, resemble each other so closely that 

 their families are apt to confound one with the other even up to 

 extreme old age. In these cases the mosaic is not only made on the 

 same design (as is the case with all beings of the same species), it is 

 not only formed from blocks taken at random, one half each from 

 two different sets and distributed at random, in two different stages, 

 but it is the same choice — I should say the same expulsion of the polar 

 globule — which has presided at the distribution of the blocks. It is, 

 as it were, a perfect replica of the same mosaic. You see, ladies, how 

 the knowledge of these laws and the mechanism of heredity explains 

 certain facts up to this time considered as unaccountable curiosities. 



It is not alone in the interest of curiosity to understand the laws 

 of heredity. I have told you that certain diseases — family diseases — 

 are obedient to these laws in their transmission; that the physical, 

 intellectual, and moral faculties are inherited in the same way. The 

 knowledge of these laws permits the avoidance of certain dangers 

 that could result from certain unions. Up to the present time doc- 

 tors are contented with dissuading in a general and vague way from 

 a union with families where one of its members might have shown 

 a physical, psychical, or pathological defect (but what family is 

 entirely exempt from it), and to proscribe all consanguineous unions. 

 Henceforth the prohibitions should be more precise and more clearly 

 stated, and, on the other hand, permits which would have terrified 

 the ancients should be given without any hesitation. Studies of this 

 kind are but just beginning. Some influential societies have been 

 formed in other countries for the study of this subject ; in England 

 and the United States they have become of great importance. In 

 France La Societe Francaise d'Eugenique was very recently organ- 

 ized. The results already obtained from these efforts are most 

 encouraging. 



In closing this lecture I wish to call special attention to certain 

 conclusions of practical importance. 



First. In microbic diseases, and especially in tuberculosis, heredity 

 is far from being fatal. One can hardly say that children of tuber- 

 culous parents inherit fi'om a soil favorable to the development 



