22 STATISTICS OF SEX 



A case <>f i wins <r of triplets has its analogue in the case of two or 

 throe particles emerging from the conduit in contiguity. They arc more 

 likely to keep together and enter the same eddies than if they were widely 

 separated in the beginning. To speak with numerical exactness there is 

 a probability of 0.77 that they will pass on the same side of the promon- 

 tory and of 0.23 that they will separate. In the case of triplets the cor- 

 responding probability would be 0.79; but these are only probabilities. 

 At any moment any two particles may widen their distance and be drawn 

 into different parts of the stream, never to reunite. 



We may thus say that the question, which branch of the river a particle, 

 emerging from the conduit, is to flow into, will be determined by a series 

 of accidents tending in one direction or the other ; and the most plausible 

 conclusion from the statistics of twins is that sex is determined in an 

 analogous way. 



7. INFLUENCE OF THE AGE OF THE PARENT ON SEX. 



The changes produced by age in the human system are such that we 

 may most plausibly look to them as causes affecting the sex of offspring. 

 The question of the influence of the age of the parent has been studied 

 by several investigators, especially by Eosenfeld, Sadler and Bertillon. I 

 have not been able to refer to the original work of Bertillon and shall 

 therefore confine myself to citing, in its proper place, one of his conclu- 

 sions bearing on the case. Dr. Eosenfeld gives the following table of the 

 sexes of more than thirty thousand births in Vienna, arranged according 

 to the age of the father. I add the percentage E m for each age : 



VIENNA STATISTICS or BIRTHS. 



Age of father Male children Female children 100 M : F E m 



Under 25 S73 767 113.7 6.5 



25 to 30 6,090 5,717 106.5 3.3 



30 35 11,987 11,291 106.2 3.1 



35 40 3,605 3,559 101.3 0.6 



40 50 622 502 128.9 10.7 



Over 50 



The table shows a decided preponderance of male children in the case 

 of young and old fathers as compared with those in middle life. The 

 conclusion thence drawn is that male unisexuality is at its maximum in 

 young and old people. 



Francke, from the statistics of Norway, reached the same conclusions 

 as regards young fathers, but the opposite as regards old ones. His 

 numbers for the ratio of male to female births, arranged according to 

 the age of the father, are as follows : 



