134 THE PHYSIOLOGY OF TWINNING 



of this view is that the same mechanism would presum- 

 ably persist throughout the reproductive life of a given 

 mother and she should always produce twins. Such a 

 condition, however, does not prevail, for almost without 

 exception mothers of one-egg twins have also single 

 children. It is barely possible, however, that cases of 

 single offspring from parents exhibiting one-egg twinning 

 are not true single offspring but that each is the survivor 

 of a pair of twins, one of which has succumbed to the 

 ever-present hazards that prevail especially in connec- 

 tion with one-egg twinning. This particular explana- 

 tion is the one that was adopted for the armadillo case, 

 where prenatal mortality is extremely low. On the whole 

 this seems the least objectionable causal theory of 

 twinning in man. 



3. A third possibility is that twinning is a hereditary 

 character dependent upon a recessive gene. The effect 

 of this gene would have to be thought of as an unfavor- 

 able growth-retarding factor that causes a temporary 

 ''period of quiescence" like that in the armadillo, 

 resulting in belated placentation and twinning. The 

 cause of twinning, according to this theory, is purely 

 intrinsic, unaffected by environment, and could be as 

 readily transmitted through the sperm as through the 

 egg. If two individuals heterozygous for the twinning 

 gene mated, some of the zygotes would be homozygous 

 for the character and twinning would result. This theory 

 would account for the fact that in twinning families there 

 may be some single offspring. This genetic theory of 

 twinning seems to me on the whole somewhat fantastic, 

 but it can hardly be excluded as one of the possibilities, 

 especially in view of Davenport's discovery. 



