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THE AMERICAN NATURALIST [Vol. XLI 



of Chicago computed upon the living births only for a given 

 period, and upon all births for the same period. It does not, 

 therefore, appear necessary to assume with Strasburger that the 

 sexual ratio for England has changed to any considerable extent 

 in two hundred years. 



The effect of a war upon the sexual ratio.— It has long been a 

 current belief that more males were born in a period following a 

 war than in a similar period of peace. Newcomb considers this 

 statement unworthy of serious consideration. It may be said 

 that, so far as the United States is concerned, such statements are 

 based upon an insufficient number of births, and that the statistics 

 are for the most part worthless. I have many letters from state 

 health officers to the effect that there are now no reliable statistics 

 of births in their respective states. 



The sexual ratio independent of external conditions. — In view 

 of the remarkable constancy of the sexual ratio under diverse 

 social, poUtical and material conditions and for long periods of 

 time in different races, it seems incredible that the determination 

 of sex should be dependent upon external conditions. 



If the sex of the offspring is independent of external conditions, 

 what is the determining factor? Two general explanations are 

 open. There is first the possibility that sex is determined by a 

 series of accidents, as Newcomb suggests, and second, the possi- 

 biUty that sex is hereditary. 



The possibility that sex is determined by a series of accidents. — 

 Newcomb likens the sex of a child to a particle floating on a 

 stream of water. In the early part of its course the stream is 

 single, but an obstacle divides it into two at the lower part. A 

 particle entering the stream at the upper part may pass on either 

 side of the obstacle, the exact course depending upon a multitude 

 of accidental causes up to a certain point, after which its course 

 on one side of the barrier or the other is fixed. So with an ovum. 

 In its early development, there is the possibility of developing into 

 either a male or a female, the sex depending upon a series of acci- 



Newcomb showed from statistics that the i)r(.l.ahility that twins 

 will be of the same sex is .77, and the pro})ahiiity that tlun- will be 

 of opposite sexes is .23. It is impossible to tell froni Newcomb's 



