THE CYTOGENETICS OF HYBRIDS 373 



acters of organisms may involve Mendelian recombinations of genetic 

 factors, or genes, actual mutations of these genes, alterations in the 

 number and assortment of chromosomes as results of aberrations in 

 mitotic behavior, and hybridization, by which many new combinations 

 are produced. Any stable sexual type is characterized by a chromosome 

 complement consisting of a certain number and assortment of elements 

 showing individual functional and morphological differences; all of them 

 constitute a balanced, harmonious system. Any change in the constitu- 

 tion of this system, whether quantitative or qualitative, large or small, 

 is accompanied by a corresponding alteration of the type, although this 

 alteration is not always immediately visible. 



With regard to the role of genes and their mutation in the origin of 

 species, Morgan (1923c) points out that the inheritance of the results 

 of gene mutations as Mendelian characters suggests the origin of 

 Mendelian differences in general in such mutations. "It is . . . now 

 clear that these smaller mutant variations must be those small heritable 

 variations that Darwin himself appealed to as furnishing the materials 

 for organic evolution. " These must not be confused with non-inherited 

 environmental variations. In reply to the objection that changes due to 

 gene mutations are too trivial to be of significance in evolution, Morgan 

 further points out that the minor superficial characters used by the genet- 

 icist and the taxonomist may often be secondary results of changes which 

 have some invisible but more fundamental physiological effects with an 

 actual survival value. Moreover, "useless" characters may be closely 

 linked with "useful" ones. The systematist of necessity distinguishes 

 many species on the basis of characters which have played no part in the 

 actual production of those species. 



Change of type is effected not only by such mutations of genes and 

 their Mendelian recombination through normal chromosome distribu- 

 tion but also by more extensive and less orderly alterations in the grouping 

 and interaction of inheritance materials through abnormalities in chromo- 

 some behavior. Non-division, non-disjunction, translocation, and other 

 aberrations all play roles in the production of new chromosome comple- 

 ments which differ from the original complements in the number, size, 

 and assortment of their constituent elements. The range of variation is 

 greatly increased by crossing among the altered types. It seems abun- 

 dantly evident that hybridization involving types with chromosomes 

 that are in some measure incompatible is one of the chief causes of chromo- 

 somal aberrations leading to the establishment of qualitatively new gene 

 systems. The irregularities in chromosome behavior immediately 

 following such crossing gradually disappear in subsequent inbred genera- 

 tions through the elimination of unpaired chromosomes, until there may 

 remain a chromosome complement whose members act in harmony, 

 although they may constitute a very new type of interacting system. 



