THE DETERMINATION OF SEX. 39 



how there is always produced a majority of the sex that is 

 wanted. That is to say, if one sex be in the decided minority, 

 or under conditions which come to the same thing, then a 

 majority of that sex will he produced. If there be, for instance, 

 a great majority of males, there is the greater Hkelihood of the 

 ova being fertilised early, but that means a probable pre- 

 ponderance of female offspring, and thus the balance is 

 restored. It would be rash to say that in every case he makes 

 out his contention, but his general argument, that disturbances 

 in the proportion of the sexes bring about their own compensa- 

 tion, is carefully and convincingly worked out. 



§ II. Sex of Twins. — It sometimes happens among many different 

 classes of animals that from one ovum two organisms develop. We have 

 then a case of " true " twins, as opposed to cases where multiple offspring 

 do not arise from one ovum. Such "true" twins seem to occur not 

 uncommonly in the human species, and are either most markedly similar 

 to one another or strongly dissimilar. The import of this is one of the 

 minor problems of heredity, and cannot be here discussed, but we have to 

 note the general fact, which holds without exception in the human species, 

 that " true " twins are of the same sex. 



From a very early date an exception to this rule has been known in 

 regard to cattle, and applies to some other organisms as well. From the 

 careful researches of Spiegelberg and others, it appears that in cattle {a) 

 the twins may be both female and then both normal, or {b) that the sexes 

 may be different and normal, or {c) that both may be males, in which case 

 one always exhibits the peculiar abnormality known as a "free-martin." 

 The internal organs are male, but the external accessory organs are female, 

 and there are also rudimentary female ducts. No theory has yet explained 

 the facts of this case. 



It is now necessary, with Diising for transition, to pass from 

 the historical mode of treatment to something more con- 

 structive. Leaving mere hypotheses behind, as well as theories 

 based on insufficient statistics, an induction from experimental 

 evidence will be built up in the following chapter. 



