DEPARTMENT OF BOTANICAL RESEARCH. 69 



of which have been proven repeatedly to breed true on first appear- 

 ance. Thus in plate 3, figures 6, 7, 8, 9, and 10, there are shown 

 certain of these new mutant forms which have appeared. Each of 

 these forms is, to a considerable extent, simply the combination 

 product of factors which were present in the original parental 

 stocks, but in almost all instances placed in new arrangements. 

 The stem-form, however, does not change and may or may not 

 throw any or all of these types, depending largely upon the condi- 

 tions of the medium. Thus, for example, a stem-stock which threw 

 these types abundantly during the latter part of the season of 1912 

 at Tucson does not, under ordinary conditions of the breeding- 

 quarters at Chicago, throw them at all thus far during the breeding- 

 season of 1913, although all are inbreeding and there is no selection, 

 so that there is produced an opportunity to bring about combinations; 

 and if these mutants are the product of Mendelian operations no 

 such effect has appeared. The total result of this type of operation 

 shows most conclusively that this is a true method of producing 

 mutating periods in organisms. In all of the series it is apparently 

 thus far always due to, first, the production of a complex product by 

 hybrid synthesis, and this must be followed later by the incidence 

 of some external factors which then act to break up the synthetic 

 strain, to give, during a period of variable duration, the behavior 

 which de Vries discovered and described as ''mutation." This 

 gives a distinctly different conception of what mutation is and its role 

 in the evolution of organisms; and in figure 1, of the plate, is shown in 

 diagrammatic fashion the difference between de Vries's hypothetical 

 mutation hypothesis and the experimental production of the muta- 

 tion behavior in these Tucson experiments. 



In the case of de Vries's hypothesis there is assumed to exist at the 

 start a species, or at least an elementary species, which was breeding 

 true, and that this began to elaborate within its germ-plasm pangenes 

 which were the bearers of new characters. These, after a sufficient 

 period of time, became sp numerous in the germ-plasm that the 

 organisms could no longer keep their action suppressed, with the 

 result that with a sudden jump they appear in the mutant form, and 

 that this mutating behavior appears through a series of generations. 

 De Vries's supposition that this behavior was probably ultimately the 

 product of external conditions seems thus far not to have been proven 

 in any of the experimental work, because when external conditions 

 are brought to bear upon a normal homozygous stock they have thus 

 far produced only a single response that is permanent in character, 

 but which was not repeated in subsequent generations. At Tucson 

 two or more stem-stocks are brought together and allowed to inter- 

 breed freely, and these eventually produce a stable form through a 

 series of synthetic operations. These stable stem-forms are the 



