Monkey Tails and Other Superfluities 205 



Moreover, there occur from time to time human beings, 

 males as well as females, in whom are present two or three 

 pair of supernumerary mammary glands. These are some- 

 times spoken of as " reversive " or "atavistic"; but such 

 characterizations assume that we understand something of 

 their causation. These supernumerary mammary glands seem 



Fig. 52. The Tail of Man 



During the early stages of development every human being has a tail that projects well 

 beyond the beginnings of the legs. During development this region is usually overgrown 

 so that at birth there is no sign of the appeiidage. Occasionally, however, a child is 

 born with a distinct tail. The one shown here was a twelve-year-old boy from French 

 Indo China, with a tail nine inches long. From drawing by Dr. Adolph H. Schultz. 

 Embryo after Conklin, Heredity and Environment, published by Princeton University Press. 



to have significance for an evolutionary interpretation since 

 several pairs of glands normally occur in the embryo of each 

 individual, and in most cases all except the pair on the breast 

 disappear. These facts, taken together, leave something to 

 explain unless we assume that in the course of time a progres- 

 sive specialization from many pairs of glands to a few or one, 

 and a progressive differentiation of maleness and femaleness, 



