502 



HEREDITY AND SEX 



His tentative conclusions are : — 



(i) The preponderance of male over female births probably 

 varies with the race. Although remarkably uniform in all 



Fig. 48. — Diagram illustrating Determination of Sex. (After Simon 

 Newcomb.) 



" Let A be a large pipe or aqueduct, from the mouth (B) of which a stream flows into 

 gradually widening river, V. At a certain distance below the exit B the river is divided into 

 two branches by a promontory, P. On one side of this promontory which we may call the 

 male side, the river is slightly broader than on the other. Between the exit and the pro- 

 montory, the river flows over a rough bottom with many eddies, but the ultimate result 

 must be that every drop of water which comes from the conduit ultimately passes on one 

 side of the promontory or the other. But the side on which it shall pass is not determined 

 at any one moment. As a drop, or, to give the analogy a more complete form, a small particle 

 suspended in the water, leaves the conduit, it is equally likely to pass into one branch of 

 the river or the other. If it chance to incline to the right after leaving the conduit, there 

 will be a greater probability of its passing into the right branch, but this will be only a pro- 

 bability until a certain point of the course is reached. A particle reaching the point M, 

 for example, will be likely to go into the female branch, but yet may be carried by an eddy 

 across to the opposite side before it reaches it. One at N, although farther down, will still 

 be uncertain ; possibly its course may not be decided until it almost reaches P. A particle 

 on one bank or the other will be more and more likely to pass into the corresponding branch 

 the farther down it is found. When the particle once crosses one of the dotted lines PR and 

 PS the branch it will take will be completely determined. 



" A case of twins or of triplets has its analogue in the case of two or three particles emerging 

 from the conduit in contiguity. They are more likely to keep together and enter the same 

 eddies than if they were widely separated in the beginning. To speak with numerical ex- 

 actness, there is a probability of 077 that they will pass on the same side of the promontory 

 and of 0-23 that they will separate. In the case of triplets the corresponding 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, vvdll 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." 



branches of the Semitic race, it seems to be either non-existent 

 or quite small in the Negro race. 



There are no important differences as regards capacity for 



