THE METHOD OF EUGENICS 21 



The lessons that this enforces are: first, that characters 

 are often and, indeed, usually, inherited independently 

 and, secondly, that the outcome of a particular mating 

 may be predicted with some precision; indeed, in many 

 matings with certainty. 



This study might be extended to cases of three or more 

 independent characters but the tables in such cases become 

 more complex and httle would be gained by making them 

 as the principle has been learned by the cases already 

 given. In view of the great diversity of parents in respect 

 to their visible characters the variability of children is 

 readily accounted for. 



5. Heredity of Sex and of '^Sex-limited" 



Characters 



In most species, as in man, there are two sexes, and 

 they are equally numerous. For a long time this equahty 

 has been a mystery; but of late years, through the studies 

 of McClung, Wilson, Stevens and Morgan, the mystery has 

 been cleared up. For there has been discovered in the 

 germ plasm a mechanism adequate for bringing about the 

 observed results. We now know that sex is probably 

 determined strictly by the laws of chance, like the turn of 

 a penny. The cytological theory of the facts is as follows. 

 One sex, usually (and herein taken as) the female, has all 

 cells, even those of the young ovarj'-, with a pair of each 

 kind of chromosome, of which one pair is usually smaller 

 than the others and more centrally placed. The chromo- 

 somes of this pair are called the X chromosomes. In the 

 male, on the other hand, the forerunners of the sperm cells 

 have one less chromosome, making the number odd. This 

 odd chromosome [exceptionally paired] is usually of small 

 size and is also known as an X chromosome. In the cell- 

 division that leads to the formation of the mature sperm- 



