180 THE MENDEL JOURNAL 



the inheritance of the twinning tendency. In the first 

 pedigree (A.) it will be seen how, in the family stock dealt 

 with, the twin births tended without exception to be 

 produced at or towards the end of the family. This is 

 certainly the case in some clanships, twins being borne by 

 multiparas whose fecundity is tending towards exhaustion;* 

 but in others this is not so, and taking all cases together, 

 the incidence is pretty much as might be expected were 

 the liability to twinning spread equally over all the births, 

 the divergencies met with in any given series being, on the 

 whole, such as might reasonably be expected to occur in 

 random samples of small size. 



Apart from statistics it may be stated in a general way 

 that twins tend to be born when the mother is advanced 

 in j^ears, and that where it occurs otherwise there will 

 probabl}^ be found to have been a strong hereditary 

 tendency to twinning either among the women's own 

 relations or among those of the father of the children. 



In the thirteen households dealt with in pedigree B. 

 there are six families in which twins are present, and such 

 are seen to have occurred altogether eight times in these 

 six families. In one of these families the tendency to the 

 plural births has been supplied through the male parent : 

 probably here in each instance the twins arose from a 

 single ovum, the male germ-cell having in it some property 

 capable of inducing want of cohesion between the two cells 

 which result from the first division of the fertilised ovum, 

 these two separated cells then going on in independent 

 development to the formation from each of a new in- 

 dividualf. From the identit}^ of the germ-plasm from 

 which they thus arose these twins were of the same sex in 

 two of the three cases in this family, though it is certainly a 



* Another way of looking at the question is to consider the relative 

 liabilit}' to twinning at the first birth in the case of women at different ages, 

 and it would appear that the more elderly a woman is the greater is the 

 liability for her to begin by bearing twins. " Prinzing gives the percentage 

 of twin labours in elderly primiparae as 4 -14 per cent. The percentage is 

 2 • 96 in primipariB under 20. 3 • 54 in women from 20 to 25, and 3 • 9 in women 

 from 25 to 30."— Brit. Med. Journ., April 20, 1912, p. 912. 



t In connection with certain of the details on this chart it may be re- 

 marked that, as I have elsewhere pointed out (Glasgmv Medical Journal, 

 June, 1910), there are reasons for believing that the tendency to twin 

 births is much more marked in families in whose composition there runs a 

 strongly inherited neuropathic strain than it is among the general population. 



