120 THE AMERICAN NATURALIST [Vol. LII 



may we safely assume the occurrence of the necessary 

 factor mutations? The fact that such mutations are 

 known in many species and that in DrosophUa the same 

 mutations have arisen anew in the same loci of homol- 

 ogous chromosomes of different pure strains would cer- 

 tainly indicate that we may. A factor mutation probably 

 involves some sort of change within the group of similar 

 molecules occupying a particular locus in a particular 

 chromosome. Obviously the number and direction of the 

 changes possible in such an entity are limited and the sum 

 of the limits of change in all the loci in the chromosome 

 group of a given species would define the limits of factor 

 mutations for that species. The limits and direction of 

 these mutations must have some bearing and may have 

 intimate bearing upon orthogenetic trend. 



Factor mutations produce new morphological and 

 physiological characters such as distinguish the forms, 

 races or varieties of existing species. As they occur 

 generally in animals and plants at the present time, we 

 may safely assume that they have occurred in preexisting 

 organisms more or less frequently, and, therefore, that 

 they have played a definite role in evolution. But just 

 how extensive is this role ? Can we account for the whole 

 process of organic evolution including the origin not 

 only of species, but also of genera, families, orders and 

 phyla upon the basis of factor mutations ? To be worthy 

 of serious consideration a theory of evolution must ac- 

 count for the development of the organic world as we 

 know it at present. Can the hypothesis of evolution 

 through factor mutations fulfil this requirement? 



It is well known that in many genera some of the species 

 differ in their chromosome number. Do factors play a 

 role in determining chromosome number? It is possible 

 that they do. It is conceivable that a factor mutation 

 might arise which would so alter the physico-chemical re- 

 lations between different parts of the chromosome as to 

 cause the chromosome to break at some point. Yet 

 chromosomes are genetic units of a higher order than 



