380 EXPERIMENTAL STUDY OF INHERITANCE 



tuations. And it may be that small fluctuations, not in them- 

 selves demonstrably heritable, may, in the course of generations 

 of consistent selection, be summed up in heritable mutations. 

 This would not exclude another possibility that mutations are 

 due to deeply saturating environmental influence. 



Hybridisation in General. — It is not desirable to attempt to 

 draw any definite line between the various kinds of crossings — 

 which may all be arranged on an inclined plane — for they differ 

 simply in the degree of difference between the two parents. We 

 may conveniently use the word " hybridisation " (cross-breeding, 

 outbreeding, exogamy) whenever there is a marked difference 

 between the two parents. The cases may be arranged on an 

 inclined plane. 



Different genera. 

 Different species. 

 Different subspecies. 

 Different breeds. 

 . . Mutants. 

 Variants. 

 Apparently identical forms. 

 Self-fertilisation (autogamy). 

 Parthenogenesis. 



Examples. — Individuals belonging to different genera — e.g. 

 domestic fowl andpheasant, sea-urchins, different genera of orchids. 



Individuals belonging to different species — e.g. capercaillie 

 and black grouse, carrion crow and hooded crow, different 

 species of Saturnia, different species of Medicago. 



Individuals belonging to different subspecies — e.g. maize. 



Individuals belonging to different breeds — e.g. poultry, 

 Short-horn and Aberdeenshire Angus cattle, Clydesdale and Shire 

 horses, silkmoths. 



Individuals belonging to different " varieties " which have 

 not risen to the stability of " breeds " — e.g. wheat susceptible 

 and immune to rust. 



Hybridisation of Distinct Species. — The conception of species 



