176 HEREDITY AND EUGENICS 



can onh^ be touched upon here. A full bibliography 

 is given b}- Wilder (191 6). The results have shown 

 that not onh^ the ridge patterns of the linger tips, 

 but also those of the palms of the hands and the soles 

 of the feet, tend to be inherited. Wilder gives some 

 striking instances of pecuhar and unusual palm and 

 sole patterns appearing in parents and offspring. 

 Thus, a man with a calcar loop on both feet married 

 a woman with a calcar loop and a divergence. Of 

 their three children, one has a loop on both feet, two 

 have a loop on one foot and a divergence on the other, 

 the loop being on the right foot in one case and the 

 left in the other. Yet the calcar loop is so rare as to 

 have been found only four times (aside from this 

 famil}") in 1,000 or more individuals examined. 



Twins. 



In the case of duplicate or identical twins (Wilder, 

 1904) these patterns show great similarit}'", though 

 not identity, and there is a distinct tendenc}^ for 

 mirror-image patterns to appear, especially on the 

 forefingers. Light has been thrown on the subject 

 of inheritance in twins by the studies of Newman 

 and Patterson (191 1, 191 6) on development and 

 variation in the armadillo. This animal always 

 produces four \^oung at a birth (some species more), 

 and these are invariably of the same sex, and have 

 been shown by Patterson (191 3) to arise by a process 

 of embryonic budding in the embryo derived from 

 a single egg. 



Stockard (1921) has recently suggested an ex- 

 planation of this condition of polyembryony in the 

 armadillo. The Texan species invariably produces 

 four young at a birth, a South American species 

 regularl}^ produces eight, while in certain species as 

 many as twelve 3^oung may be produced, apparent ty 



