36 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



assistance ? That variation does issue a new species, and that natural 

 selection is a factor, though not the only factor, in determining results, 

 is, in my opinion, as certain as that grass grows although we cannot 

 see it grow. Furthermore, I believe I have found indubitable evidence 

 of species-forming variation advancing in a definite direction (ortho- 

 genesis), and likewise of variations in various directions (amphi- 

 genesis). If I am not mistaken in this, the reconciliation for natural 

 selection, and orthogenesis is at hand." 



In concluding this brief account of orthogenesis, it should be said 

 that definitely directed evolution is now believed to be one of the laws 

 of organic evolution, but that we have no clear ideas as yet as to what 

 are its underlying causes. Therefore orthogenesis is not a causo- 

 mechanical theory of evolution at all. 



MUTATION OR HETEROGENESIS THEORIES 



The theory of "mutations" is associated with the name of Hugo 

 De Vries, the well-known Dutch botanist; that of "heterogenesis," 

 with the name of H. Korchinsky, a Russian. 



Though Korchinsky anticipated De Vries by several years, his 

 work was not supported by the large amount of experimental data 

 that characterized that of the great Dutch worker. The relative 

 claims for recognition as the founder of the mutation theory are 

 almost on a par with those of Darwin and Wallace for the natural- 

 selection theory. Both Darwin and De Vries held .back their theo- 

 ries until they appeared to be adequately supported by personally 

 collected facts. 



There is a striking parallelism between the ideas and conclusions 

 of De Vries and those of Korchinsky, and since this is true a resume of 

 De Vries's better-known work will serve to give the essentials of the 

 whole conception. 



De Vries began his genetic experiments by a study of the variations 

 of plants in the field. After learning their normal variability in 

 nature, he transferred them to the experimental garden and there 

 attempted to improve them by selection. He found that the improved 

 living conditions due to better soil and cultivation induced a wider 

 range of variability in size, luxuriance, and fecundity. Such variations 

 were plus or minus in their character, fluctuating about a mean or 

 average. It was exactly this type of variability that Darwin empha- 

 sized as the raw material of evolution; but De Vries found by experi- 

 ment that selection had no permanent hereditary effect when based 



