THE AMERICAN NATURALIST [Vol. LI 



The large amount of data which Morgan and his asso- 

 ciates have collected within the past six years has clearly 

 demonstrated that the chromosome mechanism furnishes 

 a basis upon which the behavior of Mendelian units may 

 be logically and consistently explained. In an investiga- 

 tion of Drosophila over a hundred factor mutations have 

 been discovered and studied, and these have been found 

 to fall into four groups with respect to the linkage rela- 

 tions they display with one another. These four groups 

 correspond to the four pairs of chromosomes. By deter- 

 mining the linkage values which are displayed within 

 groups it has been possible to demonstrate that there is a 

 consistent, invariable, linear arrangement of factors 

 within the chromosomes at some time in their history. 

 From this data Morgan and his associates have been able 

 to prepare a map of the relative positions of the factors 

 in the chromosomes. The complete conception, therefore, 

 pictures the chromosome at some stage in its history as a 

 linear series of loci. When a change occurs in some locus, 

 a corresponding change of some sort may occur in soma- 

 togenesis, so that the individual which develops from such 

 a set of factors with the changed locus ditfers in some 

 particular way from an individual which develops from 

 the normal unchanged series of loci. The change in the 

 characters of the individual will depend not only upon the 

 particular locus which has been changed, but also upon 

 the particular way in which that locus has been changed. 

 A changed locus, however, maintains the same position 

 with reference to the other loci as did the unchanged 

 locus, and this fact is the basis of Mendelian behavior, for 

 knowing the behavior of the chromosomes in reduction, 

 it enables us to gain a clear conception of the nature of 

 Mendelian segregation. 



When now we consider the particular factors them- 

 selves, the changed loci of the system, we see clearly that 

 important physiological relations exist among the various 

 loci. It is an appreciation of this fact that has led many 

 investigators, among them Conklin (1908), Jennings 

 (1914), Morgan (1915 5), Pearl (1915), and Wilson (1914), 



