THE ORIGIN OF VARIATIONS 227 



produced, as well as females with red eyes and males of both 

 types. 



This feature, eye color, therefore is not sex-limited but is 

 said to be sex-linked, i.e., connected with the sex chromosome and 

 is distributed with the distribution of the sex chromosome. 



Prof. Morgan has found no less than twenty-five of these sex- 

 linked factors all of which have been worked out experimentally 

 on the fruit fly and all conforming to the case illustrated above. 

 Other characteristics have been found which have nothing to 

 do with the sex chromosomes but are bound up with others. 

 These results thus appear to be a brilliant confirmation of Weis- 

 mann's hypothesis of the constitution of the germ plasm. 



F. THE ORIGIN OF VARIATIONS 



The results described above from cytological and experi- 

 mental work would seem to indicate that variations would be 

 extremely difficult to originate. If the characteristics of the 

 adult are contained in the germ plasm then the individual is pre- 

 ordained and the germ plasm would pass on to descendants with 

 the same characteristics. The individual which develops may 

 change by reason of environmental influences in many somatic 

 characteristics but how is it with the germ plasm and the char- 

 acteristics of the parents? Examples from mutilations would 

 seem to bear out the Weismann view that somatic changes 

 of the individual have no effect on the germ plasm which that 

 individual carries and transmits. Nevertheless variations do 

 arise, are transmitted by inheritance and fostered or obliterated 

 by natural selection. How they arise is still a matter of specu- 

 lation more or less founded on fact. Amphimixis, mutation, 

 inheritance of acquired characteristics, are upheld by various 

 biologists as accounting for the origin of variations. 



Amphimixis. The union of germ cells brings together 

 (amphimixis) the traits of two lines of ancestry and with the 

 union a possibility of different combinations in the segregation 

 of characteristics. Or by such union recessive characteristics 

 may be brought out, leading to divergent types in the race. 



