122 THE BIOLOGY OF'TWINS 



No experiments of the sort, carried out upon sheep by 

 Bell have been made upon cattle. Doubtless similar 

 results could be obtained, although the normal percent- 

 age of twins in cattle is very much smaller than in 

 sheep. Even if the ratio of twin births in cattle could 

 be increased by selective breeding, the advantages of 

 such an increase would be considerably reduced by 

 the frequency of sterile freemartins. 



There are, however, certain rather indirect evidences 

 that dizygotic twinning is hereditary in cattle. Several 

 cases of cows producing several sets of twins have been 

 recorded by various writers. One interesting case, 

 cited by Pearl, is that of a cow that produced suc- 

 cessively three single offspring, then two pairs of twins, 

 next triplets, then a single calf, and finally the set of 

 triplets shown in the photograph (Fig. 40). The 

 middle calf is a normal male and the two outside ones 

 are freemartins. The male is a typical Guernsey like 

 the dam, but the freemartins are therefore like their sire. 

 The two freemartins are remarkably alike and are be- 

 lieved by Pearl to be monozygotic' Cases of triplets 

 are extremely rare, but Cole records seven cases among 

 303 plural births. Since plural births in cattle occur 

 in only about 2 per cent of births, triplets occur only 

 about once in 2,000 cases. Possibly many more triplet 

 gestations begin, but result in the death or early abortion 

 of one or more members of the set. 



Since no experiments have ever been performed by 

 way of selecting for a twinning strain of human beings, 

 the only evidence of a hereditary tendency in twinning 



' In another place I have shown the improbability of monozygotic 

 twinning in cattle. 



