202 THE SOCIAL LIFE OF ANIMALS 



of bi-parental inheritance and more remotely on 

 mutations of one kind or another. 



Fortunately we have in the case of the Dionne 

 quintuplets a natural experiment which deserves 

 much attention. Detailed biological studies which 

 appeared late in 1937 confirmed the general assump- 

 tion that these much-discussed babies are an iden- 

 tical set of sisters. Biologically this means that all 

 of them have come from one ovum which was fer- 

 tilized by one spermatozoan. Soon after fertilization 

 the early cleavage cells separated and produced five 

 embryos, each with identical heredity. I shall not 

 give the details of the evidence on which this con- 

 clusion is based. In addition to looking so much 

 alike that only their regular attendants can tell them 

 apart with any degree of sureness, there are simi- 

 larities in finger and palm prints, in toe and sole 

 prints, and in other anatomic details which point 

 conclusively toward a common identical heredity. 



A group of investigators from the University of 

 Toronto have been studying the social reactions of 

 the quintuplets and have reported observations from 

 the twelfth to the thirty-sixth month of their age. 

 At first the children were placed together in a play 

 pen by pairs to observe their interactions; from the 

 twenty-second to the thirty-sixth month they were 

 observed as a group. 



