124 THE BIOLOGY OF TWINS 



due rather to the father than to the two mothers. 

 How such a condition could be determined by the male 

 is difficult to imagine, but it may well be that the 

 tendency to polyembryonic development of an egg 

 might be stimulated by some peculiarity of the sperm. 

 If the twins in this case were all duplicates, this would 

 be an interesting possibihty; but the father could 

 hardly control double or triple ovulation in the mother. 

 Unfortunately no data are given as to the sex of the 

 twins in this striking case. 



It will readily be seen that monozygotic and dizy- 

 gotic twinning involve totally different situations and 

 would therefore doubtless be inherited in quite difTer- 

 ent ways, if inherited at all. Danforth is inclined to 

 believe that the ability to produce twins is common 

 to all strains, and that there is merely a variation in 

 frequency of incidence of twins in different strains. 

 The only method of settling the question is that of 

 collecting a really adequate mass of data; no such 

 collection is yet at hand. 



Both monozygotic and dizygotic (including poly- 

 zygotic) twinning are characters capable of being 

 inherited as unit characters and of being made racial 

 or specific by selective breeding. In cattle and probably 

 also in man twinning seems to be recessive and single 

 births dominant; hence a pure twinning strain could 

 be produced, if at all, only by interbreeding homozygous 

 recessive individuals, that is, males and females that 

 are the offspring for at least two generations of ancestors 

 that were themselves twins. 



