THE AMERICAN NATURALIST [Vol. LIV 



more common phenomenon than hitherto appreciated. 

 Lethal factors, it may be pointed out, are a probable solu- 

 tion of one of the mysteries of gynecology ; namely, that 

 a woman who is sterile with one husband is often fertile 

 with another, even when examination has shown no defect 

 in the spermatozoa. Similarly a husband may have no 

 children by one wife, but one or more by a second mar- 

 riage. Parallel phenomena are common in dairy cattle. 

 We conclude then that lethal factors are probably wide- 

 spread phenomena even in human germ cells, and ac- 

 count for a certain proportion of long intervals between 

 births, of early miscarriages, and of sterile unions. 



The application of the foregoing two principles of fail- 

 ure of fertilization and failure of development to the ques- 

 tion of the role of the male in twin production is now 

 fairly obvious. More eggs are laid, even without pru- 

 dential restraint, than come to development, and this is 

 true not only of eggs laid successively but of eggs laid 

 simultaneously; that is, twins that are born are the re- 

 siduum of a greater number of twins that are started in 

 their development and of a still greater number of pairs 

 of eggs simultaneously ovulated. 



The literature of gynecology is indeed full of cases of 

 blighted twins. In a fairly large proportion of all twin 

 births, one of the twins has remained at a stage of devel- 

 opment of the third, fourth, or even earlier month. The 

 fetus is often found compressed and flattened; the name 

 j is given of papyraceus twin. The number of blighted 

 twins which have been referred to in the literature 

 amounts to several score, but naturally is a very small 

 proportion of the whole. The vast majority of blighted 

 twins are simply lost unnoted with the afterbirth. A rec- 

 ord is made only of the larger blighted fetuses ; the others 

 are entirely overlooked, since search is rarely made for 

 undeveloped embryos in the afterbirth, and the birth is 

 consequently regarded as a single one. We must believe 

 that a certain proportion, perhaps a large proportion, of 

 the fraternities which show two or three twin labors inter- 

 spersed with single labors are those in which pairs of eggs 



