SEX IN LIVING FORMS 135 



Hybrids.— It is a well-known law of biology that no 

 progeny result from union of animals of different spe- 

 cies. Different varieties of races of the same species 

 may form a fertile union, the result of which is a cross 

 between its two parents, possessing some of the quali- 

 ties of each. Such a cross is called a mongrel. All the 

 varieties of dogs are produced by crossing different 

 races, and so are mongrels. The various mixed races 

 of men, such as mulattoes and half-breeds, are also 

 mongrels. The mule is the product of a union between 

 the horse and the ass, and is a true hybrid. The off- 

 spring of hybrids are sterile, almost without exception ; 

 for the reason that they do not produce mature ele- 

 ments of generation. In the mule, the zoosperms are 

 either entirely absent, or else very imperfectly devel- 

 oped; hence the fact that a colt having a mule for its 

 sire is one of the rarest of curiosities, though a few in- 

 stances have been reported. This is a wise law of 

 nature to preserve the purity of species. 



Law of Sex.— If there is a law by which the sex of 

 the developing embryo is determined, it probably has 

 not yet been discovered. The influence of the will, the 

 predominant vitality of one or the other of the parents, 

 and the period at which conception occurs, have all 

 been supposed to be the determining cause. A German 

 physician some time since advanced the theory that the 

 two testicles and ovaries produce elements of different 

 sexual character, the right testicle forming zoosperms 

 capable of producing only males, and the right ovary 

 producing ova with the same peculiarity". The left testi- 

 cle and the left ovary he supposed to form the female 

 elements. He claimed to have proved his theory by ex- 

 periments upon animals. Even if true, this theory will 

 not be made of practical importance. It is, in fact, 



