CHAPTER XXIII 



LINKAGE, CROSSING-OVER, AND THE ARCHI- 

 TECTURE OF THE GERM PLASM 



Owing to the fact that most of the important advances in our 

 knowledge of the finer structure of the heredity machine have been 

 made with the aid of the little fly Drosophila melanogaster, this chapter 

 might be dedicated to the memory of this insect. "Although, like 

 Cinderella, Drosophila comes from the humble environment of the 

 garbage can," says Walter, "yet this fly has easily outstripped all its 

 sister competitors for genetical honors, until today it stands probably 

 as the most famous experimental organism in the whole world." 



But too much credit for the genetic revelations derived from a 

 study of Drosophila must not be given to the fly, which of its own 

 accord would never have told us anything. Though the name of 

 Drosophila may now be famous all over the world, the one who made 

 it famous is Professor T. E. Morgan, who in collaboration with an un- 

 usually able corps of assistants (especially Sturtevant, Bridges, and 

 Muller) has demonstrated to the scientific world the value of co-opera- 

 tion in research. The old adage that two minds are better than one 

 has proven true in this long and arduous, and above all fruitful, in- 

 vestigation. The work done by "the fly squad," as it has been affec- 

 tionately called by fellow-biologists, has resulted in an analysis of the 

 heredity machine so detailed as to be almost unbelievable. It seems 

 too good to be true, yet the keenest critics of the work have failed to 

 find any real flaws in the intricate fabric of conceptions that has been 

 woven. The whole story of this brilliant discovery, or series of dis- 

 coveries, cannot be told in a way that would be intelligible to the lay- 

 man or even to one with only a superficial knowledge of genetics. It 

 requires long and arduous study to understand it all, and one of the 

 reasons why the ideas have failed of general acceptance is that only 

 relatively few biologists have been willing to devote to the study of the 

 data the amount of time and labor necessary fully to understand them. 



LINKAGE 



In the last chapter a detailed account was given of the sex-linked 

 inheritance of white eyes in Drosophila. This was the first of the sex- 



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