o^5 Referate. 



gated by self-fertilization the detection of these small steps is no easy task 

 and the attempt to describe them all would undoubtedly prove a futile effort." 



Biologists must tigree that there are variations that are not inherited 

 and variations that are inherited. If we call the one a fluctuation and the 

 other a mutation, we have a distinction that will stand final analysis. There 

 are a great many facts that show us that there are different kinds of mutations 

 and if a further classification furnishes a working hypothesis, by all means 

 let us make a further classification; but why cannot it be understood that 

 it is simply an hypothesis. 



Even in 1903 De Vries clearly recognized (as indeed Holmes states) that 

 mutations are of all sizes, in certain cases so small that they are obscured by 

 fluctuations. The work of Johannsen and of Jennings has shown us that 

 variation due to the immediate environment of the individual is not inherited. 

 This we may properly call fluctuating variation. When changes of environ- 

 ment radically affect the reproductive cells of the individual, we no longer 

 have a fluctuation but a mutation. Since their attention has been called 

 to it, all investigators working with growing plants and animals have noticed 

 the great frequency with which inherited changes occur. But whether we 

 should say that the appearance of a red color in a new race of flowers pre- 

 viously white is a larger change than the lengthening of a sepal one millimeter, 

 because it appears greater to the casual observer, is a question not to be 

 decided untü we know considerably more of plant physiology during devel- 

 opment. 



Furthermore, the presence and absence hypothesis with the use of "factors" 

 describes a large number of facts relating to Mendelian inheritance. The 

 fact that we do not know the nature of a factor no more affects the utility 

 of the hypothesis than does the possibilit}? that matter may be simply differ- 

 ent manifestations of energy affect the utility of Dalton's Atomic Theory. 

 Whether we say that organisms are made up of unit characters, to which 

 Professor Holmes objects, or whether we use his own term and say that 

 they are a "complex of tendencies" seems to make very little difference at 

 present. Let us hope, however, that in the near future, we shall have suffi- 

 cient knowledge for a more precise description than is understood from 

 either term. 



The author has seconded Spillman in calling attention to the fact that 

 the existence of a greater number of pairs of Mendelian characters in different 

 individuals than the gametic number of chromosomes is not a proof that 

 the Mendelian phenomena are not functions of the chromosomes, yet he 

 believes that there are grave difficulties in the way of accepting the reducing 

 division of the germ cells as the mechanism of segregation. This criticism 

 is worthy of careful consideration by investigators: but the biologist receives 

 no aid in his next assertion, that "Mendelian phenomena can be explained 

 on the basis of the sorting out of ancestral tendencies as wholes instead of 

 as unit charaters." 



Let us consider one other portion of the paper. The author states: "If 

 now it should turn out that stability is a matter of degree the last distin- 

 guishing feature of the mutation theory would be destroyed." 



This statement needs .some qualification. A distinction should be made 

 between the theoretical features that a mutation should possess and the 

 more or less tangible qualities by which it is recognized. If we bring to- 

 gether two chemical compounds a third may be formed that soon breaks 



