248 READINGS IN EVOLUTION, GENETICS, AND EUGENICS 



2. Darwin insisted upon the idea that minute fluctuating varia- 

 tions, which we now know are to a large extent non-heritable, were 

 the principal, if not the sole, materials for natural selection to work 

 upon. He knew of a considerable number of "sports" or "saltatory 

 variations" (now called mutations), but considered these too infre- 

 quent to furnish the necessary basis for selection. We now know 

 that mutations may be as small as fluctuating variations or as large 

 as "sports" and that they are of much more frequent occurrence 

 than Darwin supposed. 



3. Darwin considered all variations as heritable. He did not 

 distinguish between somatic variations and germinal variations. In 

 fact, as we learn from a study of his pangenesis theory, he considered 

 all variations as in the first instance somatic, and subsequently 

 transferred by means of gemrnules to the germ cells. Every somatic 

 variation, whether induced by use, disuse, in response to environ- 

 mental stimulus, or through mere spontaneous variability, was sup- 

 posed to be able to give off gemmules into the blood stream that 

 would carry to the germ cells the physical basis of the varying charac- 

 ter. The pangenesis mechanism is now known to have no basis 

 in fact. 



4. The natural-selection theory is based upon a mistaken concep- 

 '"'tion of the methods of artificial selection. Darwin believed, without 



having any proof for this belief, that the way in which domestic 

 varieties had been so profoundly modified at the hands of man was 

 by the conscious or unconscious selection of slight fluctuating varia- 

 tions in favorable or desired directions, and that this resulted in the 

 cumulative improvement or enhancement of the desired characters 

 over a long series of generations. Darwin supposes that the radically 

 changed conditions of domestication hasten and stimulate variability, 

 thus offering a better opportunity for selection. Transferring this 

 idea to nature, he thinks that changed natural conditions stimulate 

 variability, just as does domestication, and that this is seized upon by 

 natural selection to make for adaptation to the new environment and 

 the resultant origin of new species. 



Our modern experimental studies have shown that somatic 

 modifications due to environmental changes are not hereditary, and 

 that all of the recent domestic varieties whose origin has been observed 

 have been the result of suddenly appearing germinal variations or 

 mutations, that arrive fully formed and cannot be improved by selec- 

 tion, except that they usually need to be selected out or isolated in 



