No. 614] FACTOR MUTATIONS IN EVOLUTION 125 



lation. Thus, for example, if one factor mutation occurs 

 in one individual among say 1,000, then the probability 

 of two factor mutations occurring in the same individual 

 at the same time would be once in 1,000,000 times. More- 

 over, individuals showing even one factor mutation are 

 comparatively rare. Hence, it appears extremely doubt- 

 ful that any species have arisen through simultaneous 

 factor mutations in single individuals. 



3. Single factor mutations may have occurred in dif- 

 ferent individuals within a group either simultaneously 

 or successively. This hypothesis implies that individuals 

 possessing certain mutant characters are capable of main- 

 taining themselves in the wild state, an assumption which 

 is justified by the fact that factor mutations are known 

 which have not impaired vitality and fertility, nor re- 

 duced the general adaptability of the organism. Further 

 evidence to support this hypothesis is found in the wide- 

 spread existence of composite species. Although these 

 heterogeneous groups have been classified as species, they 

 are really aggregates of numerous distinct varieties or 

 subspecies. Indeed, their Mendelian behavior indicates 

 that many advantageous physiological characters, such 

 as earliness or lateness of maturity, resistance to disease, 

 high fecundity or possession of a certain pigment, origi- 

 nate through factor mutations. Such mutations fre- 

 quently, though not necessarily, involve morphological 

 changes also. In all except strictly autogenous (self-fer- 

 tilized) species new combinations of mutant characters 

 would occur through intercrossing, thus increasing the 

 chances of beneficial or advantageous results to the 

 species. Populations of such species consist of indi- 

 viduals of heterogeneous germinal constitution, and fre- 

 quently disadvantageous or even lethal factor mutations 

 persist in heterozygous condition, but soon make their 

 presence manifest when inbreeding or self-fertilization is 

 practised. Autogenous species, on the other hand, are 

 composed of individuals of homogeneous germinal con- 

 stitution (pure lines) which have arisen through the re- 



