1911.] 



Stable Simple Mendelian Population. 



37 



one, the likelihood of its possessors being the offspring of first cousins is 

 much greater than the frequency of first cousin marriages would lead us to 

 expect. 



The statistics available for the purpose of comparison with these results 

 are unfortunately extremely meagre, and they cannot be regarded as more 

 than approximately accurate. 



Miss E. M. Elderton has kindly placed at my jdisposal her results as to 

 the percentage frequency of consanguinity among the parents of albinos. 

 Miss Elderton surmises that, when in her statistics it is not distinctly 

 recorded that there was no consanguinity between the parents, it is possible 

 that there may have been some relationship. 



Hence she obtains two results : firstly, by including all available figures, 

 and, secondly, by excluding those families in which the presence or absence 

 of consanguinity in the parentage is not distinctly recorded. 



In the first case she finds that 14 per cent., and in the second that 

 2T6, of albinos are the offspring of first cousin and double cousin marriages. 



Now, albinism is usually regarded as arising from a recessive Mendelian 

 factor, although strict proof of this is as yet wanting. Assuming that it is 

 such a Mendelian unit character, the above results can be applied to it. 



Prof. Pearson estimates that albinism occurs in about 1 individual in 

 20,000. With this frequency, we see from Table II that if the general rate 

 of first cousin marriage is about 1 or 2 per cent., the percentage of albinos 

 that arises from first cousin marriages, as found from calculation, is of 

 about the same magnitude as that found from direct statistics. If the 

 marriages of first cousins form a greater proportion than 1 or 2 per cent, of 

 all marriages, then the calculated result becomes too great. 



It is, however, quite impossible as yet to say definitely whether the 

 percentage of first cousin marriages largely exceeds 2 or not. In any case 

 the approximate character of the other data precludes us from obtaining 

 more than a rough test as to whether the calculated and observed results are 

 of more or less of the same order of magnitude. 



The only other numerical test obtainable has been given by the data as to 

 deaf-mutism. In the American Census of 1890 it was found that there were 

 659 congenitally deaf persons in 10,000,000 inhabitants, or, about 1 person 

 in 15,000. It is probable that most of these were deaf mutes. 



Miss Elderton finds that 8 -per cent, of deaf mutes are the offspring of 

 first cousin parents. The calculated result for 1 per cent, of cousin marriage 

 lies between 8"8 and H - 9 for 16 s = 4, and between 12'6 and 17"1 for s = oo . 

 Here again the calculated result seems to be somewhat too high. Both in 

 this case, however, and in that of albinism it may very well be that there is 



