42 



THE AMEinCAX XATi'BALIST [Vol.LVI 



than the other effects. If now selection is removed in 

 regard to any particular character, these character 

 changes w^hich occur more readily must accumulate, giv- 

 ing apparent orthogenesis, disappearance of unused 

 organs, of unused physiological capabilities, and so forth. 

 As we shall see later, however, the changes are not so 

 frequent or numerous that they could ordinarily push 

 evolution in such a direction against selection and against 

 the immediate interests of the organism. 



In regard to the magnitude of the somatic effect pro- 

 duced by the gene variation, the Drosophila results show 

 that there the smaller character changes occur oftener 

 than large ones. The reason for this is again probably 

 to be found in developmental mechanics, owing to the 

 fact that there are usually more genes slightly affecting a 

 given character than those playing an essential role in 

 its formation. The evidence proves that there are still 

 more genes whose change does not affect the given char- 

 acter at all— no matter what this character may be, imless 

 it is life itself— and this raises the question as to how 

 many mutations are absolutely unnoticed, affecting no 

 character, or no detectable character, to any appreciable 

 extent at all. Certainly there must be many such muta- 

 tions, judging by the frequency with which '' modifying 

 factors " arise, which produce an effect only in^ the 

 presence of a special genetic complex not ordinarily 

 present. 



(6) The Localization of the Change 

 Certain evidence concerning the causation of mutations 

 has also been obtained by studying the relations of their 

 occurrence to one another. Hitherto it has nearly always 

 been found that only one mutation has occurred at a time, 

 restricted to a single gene in the cell. I must omit from 

 consideration here the two interesting cases of deficiency 

 found by Bridges and by Mohr, in each of which it seems 

 certam that an entire region of a chromosome, with its 

 whole cargo of genes, changed wa< lest, and also a 

 certain peculiar ease, not vet clranMi up wVich has re 

 cently been reported by Xilson-Khle; those important 



