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Indiana University Studies 



Morg'an that these graded series of mutations in eye color 

 have not occurred in the order in which they are graded. 

 White eye color occurred at a single change from the red and 

 not thru the series of intergrades from red to white. Of 

 course as Jennings says, 'There is nothing to prevent a pas- 

 sage 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." Certainly this 

 statement is true, but the chances that the changes will occur 

 in this way are rather small. The only way Jennings at- 

 tempts to meet this objection of Morgan is by saying that the 

 mutations which have been described in Drosophila are retro- 

 gressive, and that it is doubtful whether they are the kind of 

 variations by which the complex structures were built up in 

 phylogeny. If we are to believe this and throw away this 

 much of the data from Drosophila, why not throw all of it 

 away and not use part of it to support the theory of natural 

 selection ? 



I have presented my own evidence and my own conclu- 

 sions. The evidence was beyond a doubt in favor of the multi- 

 ple factor hypothesis. Yet I do not wish to generalize and say 

 that the results of selection are always accomplished in this 

 way. It seems to me it is possible that there may be some 

 truth in both of these interpretations and that there may be 

 evolution by more than one method. In fact, it has been 

 suggested by several writers recently that we are farther from 

 a constructive process of evolution than we were fifty years 

 ago. The reason is that we are not so easily satisfied now 

 as then. Bateson, in his Silliman lectures ('13), says, ''When 

 we contemplate the problem of evolution at larg-e the hope at 

 the present time of constructing even a mental picture of that 

 process grows weak almost to the point of vanishing." Pearl 

 ('17) approaches this viewpoint when he says, "The great 

 outstanding need in research on the problem of evolution in 

 general, and of selection in -particular, is more and more 

 searching investig'ations as to the cause of genetic (factorial) 

 variation." 



