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Prof. E. B. Wilson. 



the probability of their remaining in association ; in other words, the greater 

 the " strength of linkage." Hence the astounding possibility which this 

 suggests of using the " strength of linkage " as an index of the serial order of 

 the units in the threads and the relative distances between them. This, it 

 seems to me, is the most remarkable result to which these researches have 

 led, for it opens the possibility of a detailed experimental analysis of the 

 nuclear organisation — almost, we might say, of the topographical anatomy of 

 the germ-plasm. 



By the application of this method to an immense body of experimental 

 data, Morgan and his co-workers, Sturtevant in particular, have actually 

 plotted the location of most of the units in each chromosome, and constantly 

 use the diagrams thus obtained as working models for further analysis. The 

 order and relative distances of the units in each linear series, once deter- 

 mined, are found to be remarkably constant when tested by varied 

 experiments designed to this end. The practical value of the hypothesis 

 is attested by the fact that when the distance (strength of linkage) 

 between any two units, A and B, is known, and also that between B and a 

 third unit, C, the relation between A and C may be predicted with con- 

 siderable accuracy. This fact gives reality to the assumption that each 

 unit has a definite locus in the linear series (chromatin-thread), and that 

 allelomorphic units occupy corresponding loci in homologous chromosomes. 

 Further corroboration is found in the interesting phenomena presented by 

 multiple allelomorphs, of which an example is given by the four eye-colours : 

 white, eosin, cherry, and the normal or red colour, all of which belong to the 

 first or sex-linked group in Drosophila. Any two of these are allelomorphic 

 to each other, and the important fact is that all exhibit the same strength of 

 linkage with all other sex-linked units. The inference is that each of the 

 units in question must occupy the same locus in the sex-chromosome. Since 

 no two can occupy the same locus at the same time, it follows that not more 

 than two of them can co-exist in any particular female, and not more than 

 one can be present in the male. And this corresponds with the facts as 

 actually observed. 



To those not actually engaged in such investigations this hypothesis will, 

 perhaps, seem of highly speculative character. But is it more so than many 

 working hypotheses of experimental physics or organic chemistry that have 

 proved themselves fruitful in the past ? I will not pretend to answer. There 

 is no doubt that it provides us with a simple, easily intelligible and effective 

 means of handling enormous masses of intricate data, of devising new experi- 

 ments, of predicting results. Such an hypothesis, venturesome though it 

 may seem, is something more than a speculation. 



