HEREDITY 



183 



of the study of the results of heredity which to-day is being most 

 investigated, the determination of the "laws" of inheritance 

 of characteristics. 



The similarity or dissimilarity of the two mating parents is a 

 matter of much importance in regard to the results of inherit- 

 ance. To produce a fertile mating the two parents have at 

 least to be nearly allied. We are accustomed to take this for 



FIG. 115. Romulus, the striped colt of a horse mother and zebra father. 



(After Ewart.) 



granted, but the actual degree of phyletic relationship necessary 

 in fertile mating is a point of much biologic interest. In most 

 cases both parents must belong to the same species or kind, but 

 among animals and plants there have been noted exceptions 

 to this rule, these exceptions constituting the facts of hybridi- 

 zation. 



Hybridism is practically limited to mating of different 

 species of the same genera. Only in a few recorded cases have 

 organisms of different genera mated in nature with the produc- 

 tion of offspring. In zoological gardens and menageries the 



