78 HEREDITY AND EUGENICS 



of abnormals in the offspring of crosses with normals 

 (25 : 14). He concludes, " on the whole, w^hile these 

 cases give ver}- definite evidence of the segregation 

 factor, the}^ do not seem to me to favour the segrega- 

 tion in rigid Mendelian proportions." The stock of 

 this family is otherwise normal, and the}^ have no 

 difficulty in finding normal mates. The gait is un- 

 gainl}^, but the children at school hold their own in 

 writing, drawing, and needlework. Additional data 

 for this family are given by McMullen and Pearson 

 (191 3), the numbers of abnormals recorded having 

 increased from twenty-five to thirty-three. In the 

 third generation the result is in conformity with the 

 Mendelian expectation of equality of normals and 

 abnormals. In the fourth and fifth generations there 

 is a total of twenty-eight affected to thirteen normal. 

 Ever3'thing considered, it seems most reasonable to 

 regard the condition as due to a single Mendelian 

 dominant factor which arises from time to time as 

 a mutation, but which, through increased viability 

 or for some other reason, occurs in more than 50 per 

 cent, of the offspring from crosses with the normal. 

 Lewis (1909a) records certain other split-foot pedi- 

 grees. The only facts that need specially concern 

 us are : (i) a family of ten, in w^hich half are abnormal 

 and half normal; (2) a family of six, all abnormal. 

 In the latter the father may perhaps have been 

 homozygous. 



Brachydact3dy is an abnormality which has been 

 even more studied in recent 3^ears, although it is 

 probabty much less frequent in its occurrence than 

 split-foot. The condition is illustrated in Fig. 13, 

 which shows normal and brachydactylous hands for 

 comparison, and Fig. 14, w^hich is a radiogram showing 

 the bones of a brachydact^dous hand. The earliest 

 stud}^ of the inheritance of brachydactyly was by 

 Farabee {1905) in an American family from 



