Life and Death, Heredity and Evolution 



tions? In the eye of Drosophila variation may occur from 

 red to white directly, without transitional stages; or from 

 any grade to any other; the continuous scale of colors we 

 have mentioned is obtained only by arranging the steps in 

 order. Some maintain therefore that evolution has occurred 

 by such large steps, not by gradations. This conception 

 has evidently lost the compelling force it seemed to have be- 

 fore hereditary variations in minute grades were detected. 

 The very facts in such an organism as Drosophila show that 

 there is nothing to prevent a passage from one extreme to 

 another by minute changes, such as are held to occur by 

 palaeontologists and selectionists. Further, in such cases 

 as the eye-color of Drosophila we are dealing with charac- 

 ters that are already highly developed, and the changes we 

 observe are mainly retrogressive. We know, for example, 

 that the red eye color of Drosophila is formed by the co- 

 operation 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 

 completely. But it may be doubted whether this implies 

 that in the original production of this complex character, 

 with its numerous underlying functional parts, there was 

 the same change by sudden large steps. Is there any rea- 

 son to suppose for example that at one time there was a 

 complete eye save for the absence of the red color ; and that 

 this suddenly appeared? Our knowledge that this red color 

 is made by the cooperation of many diverse parts makes 

 such a notion almost inconceivable. Destructive changes in 

 a fully formed character, such as we see in the large major- 

 ity of cases of mutation, could hardly be expected to throw 

 light on how that character was built up. The observed 



