TWINS AND IIEUEDITY 31 



Ringebu families while only (),:« per cent occur in the i,'eneral ])opu- 

 lation, i. e. 4 times as many. 



We thus lind that in these families there is a great increase in 

 the number of twin-births from both one ovum and from two, and, 

 further, that this increase certainly is somewhat greater in the case 

 of those from two ova. The difference, however, is altogether too 

 inconsiderable to allow the assumption to be made, that the disposition 

 of giving birth to twins is inherited only in the case of two-egg 

 twins. Indeed, the values obtained seem to indicate the hereditary 

 nature of twin-birth both from two ova and from one ovum. 



Contrary to the other authors here mentioned Davenport also 

 supposes that the tendency to 1-egg twins is inherited. He does not, 

 however, enter upon a critic of the older studies. Davenpoht bases his 

 hypothesis principally on the following data. 



»If instead of considering the cases of twins in general we pick 

 out those of certain (or highly probable) identical twins, then we find, 

 in 30 families with such twins, that the mothers came from fraternities 

 in which (in 77 labors) there were 13 per cent twin labors, and the 

 fathers came from fraternities in which (in 38 labors) there were 13 per 

 cent twin labors. Here we see that there is an equality of the maternal 

 and paternal influence and that there is a larger proportion of relatives 

 of identical-twin producers who are twins than of producers of twins 

 in general. Indeed the occurrence of twin-offspring to the fraternities 

 of the parents of identical-twin producers is proportionally 12 times as 

 common as in the population at large.» 



This material seems, however, too limited to allow quite definite 

 conclusions. On the male side his statistics embraces ö plural births 

 among 38 labors and on the female side 10 plural births among 77 labors. 

 Further it has to be reckoned with that some of these plural births 

 may be two egged (no statements are made as to similarity or dissimi- 

 larity of sexes). 



If however any conclusions should be drawn from this material it 

 is rather that a tendency exists to inheritance of one egg twins. Con- 

 formably to what is previously stated in these pages this inheritance 

 seems also from other points of view to be very probable. 



In this connection it may be interesting to make a comparison 

 with the conditions found among mammals in general. Among most 

 of the mammals pluriparous birth depends upon the detachment of 

 several eggs. That this kind of pluriparous birth is inheritable is 

 evident. However, Newman and Paterson have recently proved that 



