34 THE EVOLUTION OF SEX. 



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 can 

 not 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 Dusing for transition, to pass from the 

 historical mode of treatment to something more constructive. Leav- 

 ing mere hypotheses behind, as well as theories based on insufficient 

 statistics, an induction from experimental evidence will be built up in 

 the following chapter. 



