OCCURRENCE OF TWINS 181 



difficulty that in the third case the sexes were dissimilar. 

 " In twin-bearing women the ovaries, according to 

 Hellin, contain an unusually large number of ovisacs, a 

 persistence, in fact, of the foetal character of the glands/'* 

 01shausen,f en the other hand, considers that the here- 

 ditar}^ tendency to twin pregnancies depends more prob- 

 ably on an unusual number of the follicles in one ovary 

 coming to maturity at the same time tlian on an abnormal 

 redundancy of ova occurring in the same follicle, which 

 is still another of the suggestions that have been made. 

 Obviously none of these reasons can hold where the 

 twinning tendency in a family comes via the father of the 

 children, as it has done in this case. 



In the third chart (pedigree C.) are to be seen four 

 distinct, and so far as is known, otherwise unrelated 

 twinning stocks, which have intermarried- so that in the 

 fourth generation of descent there is a household of three 

 children into whose composition there may be supposed 

 to enter the potentialities of all these four stocks. This 

 affinity of like for like in the selection of a mate has been 

 dignified by the name of assortative mating, and has 

 hitherto been recognised as occurring mainly in the case of 

 degenerative conditions ; persons coming from alcoholic, 

 tuberculous, epileptic, insane and otherwise neuropathic 

 stocks seeming often to have a strange perversity for 

 marrying into a family where a similar taint exists. 

 What the fundamental instinctive bias underlying this 

 phenomenon may be it is difficult to sa}^, but such a 

 selection in mating is certainly not of a conscious nature. 

 It is of interest to realise that the same tendency may exist 

 in the case of such states as we are at present consider- 

 ing, which do not fall into the category of the degeneracies. 



A further illustration of the tendency to assortative 

 mating where the tendencj^ to didymogeny exists is 

 given in the last chart of this series (pedigree D). This, 

 however, is in its way one of the most remarkable charts 

 in my collection ; the tendency to plural births here being 

 so strong that one woman had borne twins thrice in her 

 six pregnancies, while another, married to a man from 



* Cited by Ballantyne, in Allbutt and Playfair's System of Gynecology, 

 1896, p. 66. 



■fZentralblaU fur Gyndk., No. 4, 1905. 



