XII THE CHROMOSOME THEORY 139 



a linear one. Each chromosome is regarded as 

 a string of beads, each bead corresponding to a 

 factor (cf. Fig. 40). Every factor that a chromo- 

 some contains has a definite fixed position, or locus, 

 in the chromosome. One may be near the middle, 

 another near the end of a chromosome ; but wher- 

 ever it may be situated the position of any given 

 factor in the chromosome in which it occurs always 

 remains the same for all of the individuals of the 

 species. With this brief outline of the relation 



J' Black y O Grey 

 O vestigial . ■*■ normal 



Black O Xr^Grey QGrey X J Black 



.a..f;»;.il + ^-' nnrmal '*' normal 



vestigial ' 



vestigial 



-1 1 



i I 1 



Grey Black Grey Grey Black Black 



normal vestigial normal vestigial normal vestigial 



50% 50% 41-5% 8-5% 8-5% 41-5% 



Fig. 41. 



For explanation see text. 



between factors and chromosomes which the chromo- 

 some theory implies, we may proceed to discu.ss the 

 mechanism of inheritance in the case of two factors 

 which are carried by the same chromosome. 



As an example we may take the instance of 

 two pairs of characters, the factors for which are 

 both located in one of the two long pairs of chromo- 

 somes, viz. Chromosome II. These are grey, as 

 opposed to black, body-colour, and normal as opposed 

 to vestigial wings, a form in which the wings are so 

 reduced in size that the insect is incapable of 

 flight. In each of these pairs of characters the 



