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 gemmules 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 
