294 JENNINGS: CHANGES IN HEREDITARY CHARACTERS 



and the next one is not detectible. His opponent, on the other 

 hand, perhaps thinks of the series as actually continuous. But 

 the difference is not a pragmatical one; when steps become so 

 minute as to be beyond detection, the question whether they 

 exist becomes metaphysical. 



To put the case in brief, if the mutationists are to show that 

 the existence of multiple modifying factors has any bearing on 

 the general question of the effectiveness of selection, they must 

 show that such factors are not themselves minute changes in 

 the hereditary constitution. Not only have they made no 

 attempt to do this, but in the only well-examined cases they 

 state squarely that such factors are indeed alterations in the 

 hereditary constitution. 



For the inheritance of such factors as Mendelian units, of course 

 absolutely nothing is required save that the location of the 

 change is in a chromosome. No particular degree of magnitude ; 

 no unity of any other kind is required. 



But there remains one point brought out by the mutationists 

 which ; s of great importance to the student of the method of 

 evolution. While they must admit, by their own account, that 

 all these grades occur, so that a practically if not actually con- 

 tinuous series can be formed, they of course point out that the 

 changes do not occur in a continuous series. In the eye of Droso- 

 phila variation may occur from red to white directly, without 

 any transit onal stages; or from any grade to any other; the con- 

 tinuous scale is obtained only by arranging the steps in order. 

 Therefore, it is maintained, evolution may have occurred by 

 such large steps, not by continuous gradations. 10 This is of 

 course a matter deserving of serious consideration. But cer- 

 tain other points must be considered also. First, the very facts 

 known for Drosophila show that there is nothing to prevent a 

 passage from one extreme to the other by minute changes, just 

 as is held to occur by the paleontologists and selectionists, 

 although change by large steps occurs also. Secondly, in such 

 cases as the eye color of Drosophila we are dealing with char- 



10 See particularly the discussion of this point in Morgan, 1916, p. 7-27. 

 (See Bibliography.) 



