No. 614] FACTOR MUTATIONS IN EVOLUTION 117 



pretation without any necessity for assumption of exten- 

 sive germinal changes." On the other hand, Muller's re- 

 cent investigation of balanced lethal factors in Drosophila 

 led him to conclude "that some (if not most) of the so- 

 called mutations in 0. lamarckiana are but the emergence 

 into a state of homozygosis, through crossing over, of 

 recessive factors constantly present in the heterozygous 

 stock." If this is correct and these recessive characters 

 arose as factor mutations, it is obvious that, in basing his 

 theory of speciation by mutation on the evidence from 

 (Enothera, de Vries builded better than he knew ! 



During the decade following de Vries 's announcement 

 of his theory biological interest shifted from the general 

 problem of evolution to the more specific problem of 

 heredity. The rediscovery of Mendel's law at once 

 focused attention upon the inheritance of particular char- 

 acters. Then began the era of experimental evolution in 

 which, under the leadership of Morgan, most remarkable 

 progress has already been made. The traditional prob- 

 lem of heredity, its mechanism, has been solved. We 

 know, not only that the ultimate hereditary units are 

 germinal, but also that they are located in that particu- 

 lar portion of the germ cell called the chromatin, and there 

 is an ever-growing body of evidence proving that each 

 hereditary unit occupies a particular locus in a particular 

 chromosome. These hereditary units have been desig- 

 nated by various terms, but are most commonly referred 

 to as genes, genetic factors, unit factors or simply factors. 



The germ plasm has come to be recognized as an ex- 

 ceedingly complex stereochemic system, and, as Eeichert 

 has pointed out, on account of the impressionability and 

 plasticity of such a system the germ plasm must be ex- 

 ceedingly sensitive to changes in internal and external 

 conditions. That factors, however, are relatively stable 

 entities is being clearly evidenced all the time. But occa- 

 sionally they undergo definite alteration, doubtless as the 

 natural result of some new or peculiar set of internal con- 

 ditions. These alterations in genetic factors, or factor 



