HISTORICAL ACCOUNT OF EVOLUTION THEORY 37 



to fluctuating variations, since the latter were merely somatic responses 

 on variable growth conditions. This negative finding led him to 

 renewed interest in discontinuous or saltatory variations as the only 

 alternative to fluctuating or continuous variations. 



He looked far and wide among species of wild plants for a species 

 that might exhibit a significant amount of saltatory variation and 

 finally discovered in the evening primrose (Oenothera lamarckiand) 

 what seemed to exhibit exactly the hoped-for characteristics. This 

 large, stately plant with conspicuous yellow blooms had escaped from 

 cultivation and was growing wild in the fields. In addition to a large 

 number of plants that showed only minor differences among them- 

 selves, De Vries found several individuals growing among the typical 

 individuals which differed not merely in degree but in kind. These 

 were as different as distinct varieties, and, when the seeds were 

 planted in the garden they bred true to their kind. The only ques- 

 tion now was whether they had actually arisen from typical parents. 

 To test this possibility, seeds of several typical plants were planted 

 in the garden; the result being not only a repetition of the peculiar 

 types observed in the field, but of about a dozen other true breed- 

 ing types with well-marked differences from the parent-species and 

 among themselves. 



These new types De Vries considered as new elementary species 

 and he called them "mutants." They came into existence suddenly 

 in one generation and, as a rule, bred true. Whatever factors were 

 responsible for mutations, the seat of origin must have been in the 

 germ cell and not in the soma. Consequently they were inherited 

 fully from the start. The same mutations occurred in considerable 

 numbers and in successive years. In one case a given mutation 

 occurred only once in eight years of observation. Some mutants 

 were robust and successful, others were weak and incapable of living 

 under natural conditions, others were sterile. On the basis of these 

 results, which are reported in detail in chapter xxiv, De Vries came 

 to the conclusion that evolution was based upon the sudden appear- 

 ance of new varieties or elementary species and not upon the natural 

 selection of fluctuating variations. 



The mutation theory compared and contrasted with the natural 

 selection theory. It will be recalled that the raw material upon which 

 natural selection works is the minute individual or continuous varia- 

 tion that is universal in all living forms and is known to be largely 

 somatic in character and due to differences in environment. Darwin 



