232 THE PERPETUATION OF ADAPTATIONS 



grand-daughters (containing a black X and an open X) with 

 the white-eyed male s (containing an open X and no X). Two 

 white X's are brought together, and females with white eyes are 

 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, ie. connected with the sex chromosome, 

 and is distributed with the distribution of the sex chromosome. 



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

 linked factors, all of which have been worked out experimentally 

 on the fruit fly, and all conform 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 or 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 

 preordained, 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 characteristics 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 speculation, more or less founded on fact. Amphi- 

 mixis, 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 



