JENNINGS: CHANGES IN HEREDITARY CHARACTERS 295 



acters that are already highly developed. We know for example, 

 that this particular character is formed by the cooperation of 

 many separate parts of diverse chromosomes; it is a highly 

 complex product of evolution. Now, we find that one or another 

 of these parts may suddenly cease to perform its function, so 

 that the red color is not completely formed ; there is a sudden 

 change in it; or it may disappear entirely. But is this after all 

 strong evidence that in the original production of this complex 

 character with its. numerous underlying functional parts, there 

 was the same change by sudden large steps? Indeed, is it not 

 rather true that such destructive changes in a fully formed 

 character could not be expected to throw light on how that 

 character was built up? 



I am not unmindful of the fact that there are a few — but only 

 a very few — cases in which there is indication of a positive addi- 

 tion by a definite step, as when the eosin color is produced in 

 white-eyed stock. But here again the underlying apparatus has 

 before had the power to produce eosin and other colors. The 

 white color was due to the temporary suspension of function in 

 parts of the chromosomal apparatus, and it may be doubted 

 whether the restoration of this function throws light on the way 

 the apparatus was first developed. 



To sum up, it appears to me that the work on Drosophila is 

 supplying a complete foundation for evolution through selection 

 of minute gradations. The so-called "multiple allelomorphs" 

 show that a single unit factor may thus exist in a great number of 

 grades; the "multiple modifying factors" show that a visible 

 character may be modified in the finest gradations by alterations 

 in diverse parts of the germinal apparatus. The objections 

 raised by the mutationists to gradual change through selection 

 are breaking down as a result of the thoroughness of the muta- 

 tionists' own studies. We have already gotten completely rid 

 of the notion that the germinal changes consist only in the drop- 

 ping out of complete units, or that they are bound to occur in 

 large steps. If the recent rate of progress is maintained, when 

 such an organism as Drosophila has been studied for fifty years, 

 instead of eight or nine, there will be no conceivable gradation 



