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THE AMERICAN NATURALIST [Vol. LIV 



per cent, twin labors. Here we see that there is an eqality 

 of the maternal and paternal influence and that there is a 

 larger proportion of relatives of identiccd-tivin producers 

 who are twins than of producers of twins in general. In- 

 deed, the occurrence of twin-offspring to the fraternities 

 of the parents of identical-twin producers is propor- 

 tionally 12 times as common as in the population at large. 



Another way of testing the inheritableness of 1-egg 

 twins is by getting the frequency-distribution of the sex 

 of twins in repeater families— those in which the influ- 

 ence of heredity most clearly shows itself. In these, 

 therefore, we expect nearly an equality of twins of sim- 

 ilar sex and of dissimilar sex, provided 1-egg twins are 

 not found in these clearly inheritable strains. In 160 

 pairs of twins in repeater families, of which the sex is 

 given, there are 54 of unlike sex and 106 of like sex. Ex- 

 pectation in the case of binovular twins is that there will 

 be an equality of like and unlike sexed twins. Any excess 

 of like-sexed twins is to be ascribed to the occurrence of 

 1-egg twins. In the present case, there is an excess of 52 

 pairs of like-sexed twins out of 160 pairs of twins, which 

 indicates that about 1 in 3 of the twins in repeater fam- 

 ilies are identical twins, and this agrees approximately 

 with statistics obtained from the population as a whole. 

 From this we reach the conclusion that the tendency to 

 production of 1-egg twins is certainly not less common in 

 the case of repeater families than in the case of families 

 in which there is only a single pair of twins. The state- 

 ment, therefore, that there is no hereditary influence to 

 be detected in the case of 1-egg twins appears certainly to 

 be incorrect. In fact, the presence of heredity is more 

 striking than in the case of other twins and this leads us 

 to conclude that the hereditary tendencies toward unio- 

 vular multiple production so obvious in armadillo (Ta- 

 tusia) persists also in man. 



Still another way of testing the relative influence of the 

 mother and father in twin production is the comparison 

 of cases in which the father of twins has married twice, 

 and the mother of twins has married twice. An examina- 

 tion of our records showed 30 families where at least one 



