226 ESSENTIALS OF BIOLOGY 



79. OAT HYBRIDITY 



A hybrid organism is the offspring of male and female parents 

 that belong to different categories. (Again the definition is of 

 the nature of a first approximation and it will be qualified and 

 amplified in the following sections.) Let there be a hierarchy 

 of categories : 



order, family, ge?ius, species, local race 



and it will be found that there is a boundary somewhere such 

 that the individuals belonging to the categories on either side 

 of the boundary are infertile with each other. Usually the 

 individuals of a local race are fertile with those of other local 

 races that are included within the same species, but usually the 

 individuals belonging to a species are infertile with those of other 

 species that belong to the same genus. We may, to begin, draw 

 the boundary between the categories, genus and species, but 

 it is not impossible that individuals belonging to different genera 

 may be fertile with each other. This is exceptional, but evidently 

 the place of the boundary is obscure. 



The obscurity of the boundary is due to what may be called 

 *' physiological reasons " (though the statement is unilluminating). 

 It is the case that the definitions of the categories are obscure since 

 they have been based on morphological and not on physiological 

 criteria. Thus we may postulate that all the individuals of a 

 species are to be regarded as interfertile, so that if we observe 

 that the individuals are infertile the conclusion is that they belong 

 to different species. This means that we include infertility with 

 other categories as a part of the definition of a species and such a 

 criterion cannot be applied to the majority of the species of the 

 classifications, for we do not know what are the facts with regard 

 to most of these formal categories that live in the wild and have 

 not been domesticated, or made the objects of experiment. There 

 may even be infertility between the individuals that belong to the 

 same local race and it is known that wild animals may not breed 

 when kept in captivity. 



79^. Immediate and Ultimate Sterility. In general the 

 individuals of different well-known species that live in the wild 

 state are sterile with each other. It is said that there are " instinct- 

 ive antipathies " (as between dog and cat), or anatomical reasons 

 (such as mere differences in size), why such specifically different 



