374 INTRODUCTION TO CYTOLOGY 



In more general terms, hybridization, or outcrossing, plays its chief 

 role by inducing variabihty in immediately subsequent generations, the 

 amount of variability depending on the degree of constitutional difference 

 between the types involved. If free intercrossing with the parental and 

 other types continues, the variability is kept up. In order for stable 

 types to differentiate from the series of variants, crossing must be 

 restricted by some form of isolation, such as natural or artificial selection, 

 geographic isolation, intersterility, and the like, whereupon some of the 

 inbreeding types rapidly become stable. The origin of species is thus 

 frequently conceived as a fixation of stable independent types through 

 the inbreeding of variable types originally resulting from hybridization. 

 We have also seen that hybridization, if accompanied by chromosome 

 doubling, may yield new and stable types almost at once. 



Since hybridization has long been recognized as an important cause 

 of diversity in organisms, it may be well to summarize the contributions 

 which cytology has made in this connection. Cytology has shown the 

 close relation between hybridization and heteroploidy, between hetero- 

 ploidy and certain classes of mutations, and hence between hybridization 

 and such mutations. In its elucidation of chromosome aberrations in 

 hybrids it has thrown much light on the curious behavior of inherited 

 characters in hybrid lines; the apparently non-Mendelian inheritance in 

 crosses between species, varieties, and mutants, the genetic constancy of 

 certain hybrids, the unexpected gametic ratios frequently obtained, the 

 unlikeness of reciprocal hybrids, sterility, and a variety of otherwise 

 unintelligible phenomena. It has heightened the precision with which 

 certain changes in character can be attributed to crossing, often affording 

 real proof in the case of suspected hybrids. It has shown the value of 

 cytological data in determining taxonomic relationships within narrow 

 circles of affinity and the prime necessity of utilizing such data in many 

 problematic cases. It has shown the importance of apomixis and somatic 

 mutation as factors in heredity and hence their importance to the 

 geneticist and field taxonomist. In short, cytology has afforded a com- 

 mon rationale for a great variety of phenomena hitherto studied by dis- 

 tinct groups of biologists. 



