That the occurrence of mongolism in twins must be regarded as important and that it may possibly throw light on the etiology are admitted by all authors that have studied this affection. Since the publication of Fraser's1 paper, ten cases of mongolism in one of twins and two cases in both have been reported. I have been able to add to this series five new cases in which only one of the twins was a mongolian idiot.
A study of the literature reveals that almost nothing is known about the causation of mongolism. Several authors have regarded the affection as dependent upon diseases of the mother, or as acquired during pregnancy through the operation of other influences injurious to the embryo. The only fact that can be asserted as having etiologic significance is that the great majority of mongolians are the children of mothers who are nearing the end