cox, DARWIN AND THE MUTATION THEORY 449 



The point I particularly wish you to notice in this case is that Mr. Dar- 

 win was employing practically the methods now used by Professor de Vries, 

 Professor MacDougal and others who are engaged in species testing, by 

 growing naturally variable or mutating plants imder conditions of rigid 

 control, so as to exclude crossing or, as de Vries calls it, vicinism. In this 

 view of the matter, it would be interesting to know what percentage of Mr. 

 Darwin's plants exhibited the new and constant characters and through 

 how many generations his mutants were found to breed true, for then we 

 could compare his results with those of investigators of our day. But his 

 attention was centered upon the endeavor to find a cause for the abrupt 

 variations and not on the formulation of laws of their action. Apparently 

 he considered isolation to be the principal secondary cause or favoring con- 

 dition, upon which view the obvious comment is that it requires no great 

 stretch of imagination to conceive of similar isolation as occurring in nature 

 and thus favoring mutation among uncultivated forms. 



Having now hastily reviewed the oscillations in Darwin's opinions con- 

 cerning the kin^ds, the causes and the laws of variation with relation to the 

 origin of species, it is not my purpose to enter upon a discussion of the 

 present-day mutation theory, which has grown out of a closer study, and a 

 more scientific treatment, of the problems of variation and heredity than were 

 attempted, or were perhaps possible, in Darwin's time. It is desirable, 

 however, to compare Darwin's views with generahzations from the muta- 

 tion theory, which we can do, well enough for our present purpose, by merely 

 recalling the seven laws which de Vries claims to be the logical outcome of 

 his twenty years of cultural experiments upon plants. They are, with 

 slight modifications as to wording and order, as follows: 



1. New elementary species appear suddenly without intermediate steps. 



2. New forms spring laterally from the main stem. 



3. New elementary species attain their full constancy at once. 



4. Some of the new strains are elementary species, while others are to 

 be considered as retrograde varieties. 



5. The same new species are produced in a large number of individuals. 



6. Mutations take place in nearly all directions and are due to unknown 

 causes. 



7. Species and varieties have originated by mutation, but are, at present, 

 not known to have originated in any other way. 



Now, looking back over what Darwin wrote concerning variation, I can 

 not believe that he would seriously have disputed any of de Vries's propo- 

 sitions except the last. All would have had to stand or fall with that. He 

 recognized the fact that new species had sometimes appeared suddenly 

 without intermediate steps and that the new forms had sprung laterally 



