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THE AMERICAN NATURALIST [Vol. LI 



the production of right and left eyes of different sex- 

 linked colors. If, among the descendants of such unusual 

 cells, germ cells should be produced, a new type of organ- 

 ism should result from their development. Although 

 Babcock and Lloyd (1917) reject somatic segregation 

 because one supposed case of that phenomenon reported 

 from the Oregon Agricultural Experiment Station 

 proved, in their opinion, to be something else, there is still 

 some evidence, like that of Hyde's, that such unequal 

 divisions' do occur. If they occur in the line of the germ 

 cells, new modifications may thereby be produced in the 

 next generation. 



Failure of the maternal and paternal chromosomes to 

 separate in the reduction division of maturation, a phe- 

 nomenon discovered in Drosophila by Bridges (1913) and 

 named by him non-disjunction, may also be the cause of 

 occasional evolutionary^ changes. 



The foregoing chromosomal irregularities, which have 

 the appearance of being mechanical rather than chemical 

 phenomena, are not, however, necessarily the instruments 

 with which permanent changes of organization are 

 wrought. Probably they are not the usual ones. They 

 have been mentioned first because there is evidence that 

 such changes are occurring now. Minute changes far 

 below the ]>resent limits of visibility are as conceivable, 

 and in my opinion quite as probable, as the grosser ones 

 nanu'd. In wliat these changes occur no one knows, for 

 no one knows the nature of the hereditary elements. 

 Suggestions involving enzymes and side chains have been 

 made. These are only conjectures, but they reveal a 

 belief that the phenomena of inheritance are chemical 

 phenomena. If we believe that heredity is dependent 

 upon chemical processes, there seems to me no escape 

 from the assumi)tion that evolution is first of all a chem- 

 ical change. What the cause of these changes may be 

 is another question ; but if changes in considerable frag- 

 ments of chromosomes, or even in whole chromosomes, 

 can occur as a result of agencies within the organism, as 



