372 castle: bole of selection in evolution 



The latter involves a double representation of every chromo- 

 some in the cell nucleus; the lata mutant involves the presence 

 of a single extra chromosome. What chromosome changes, if 

 any, are involved in other of DeVries' mutants which do not 

 Mendelize is unknown. Morgan has shown that in Drosophila 

 a unit-character change almost certainly involves a change in a 

 definitely localized part of a single chromosome. But he ap- 

 plies the term mutation to each unit-character variation of 

 Drosophila, of which he has observed over a hundred. Some 

 of these are not at all striking, involving only a slight change in 

 the shape, size, venation, or carriage of the wing, which might 

 easily be overlooked by the ordinary observer. Many of them 

 also fluctuate. Hence it is obvious that Morgan's use of the 

 term mutation is very different from that of DeVries, its origina- 

 tor. To Morgan, mutation as illustrated in Drosophila is 

 simply change by a unit-character. With this conception of 

 mutation, Morgan attempts to combine the genotype concep- 

 tion of Johannsen. He regards unit-character variations as the 

 only kind of genotypic variations and these as fluctuating (if 

 at all) only through the interaction of other unit-characters, 

 each one by itself being incapable of fluctuation. 



It will be observed that as regards the term mutation, we 

 have a very confused state of terminology which results in 

 much discussion at cross-purposes, because persons using the 

 same term have different things in mind. 



But in this discussion, however confused its terminology, 

 there are really involved two contrasted sets of general ideas, 

 two alternative lines of explanation of evolutionary change, 

 one favored by Darwin, the other offered as a substitute by 

 DeVries and accepted by Johannsen and Morgan. We may 

 briefly outline them as follows: 



Darwin DeVries 



1. New types are for the most 1. New types are created only 



part created gradually. abruptly. 



2. New types are for the most 2. New types are fully stable. 



part plastic. 



3. One evolutionary change fol- 3. One evolutionary change has 



lows upon and is made pos- no necessary relation to 



sible by another. another. 



