XII THE CHROMOSOME THEORY 147 



caution in accepting a theory of which the conse- 

 quences are so important and far reaching. 



One further point may here be mentioned, viz. 

 the bearing of the chromosome theory upon the 

 Presence and Absence hypothesis. If each factor 

 is represented by a definite portion of the length of 

 a chromosome — if the factors form a linear series in 

 the manner of a string of beads — then it is clear 

 that the Presence and Absence hypothesis, in the 

 sense that a definite portion of the chromosome 

 length may be present or absent, cannot be supposed 

 to hold good. Otherwise a chromosome bearing 

 the factors for a number of dominant characters 

 would be longer than the corresponding one which 

 did not carry these factors. The strings that had 

 to pair with one another would be of unequal length : 

 corresponding factors would no longer find them- 

 selves exactly opposite one another, and disorder 

 would result in the process of crossing over. On 

 the chromosome theory it must be supposed that 

 each member of a given pair is always made up of 

 the same number of the portions which correspond 

 with factors. This had led to the conception that 

 the recessive character is also represented by a 

 definite factor occupying a definite portion of the 

 length of the chromosome, just as the factor for the 

 dominant character does. There is a definite factor 

 for black body-colour in Drosophila as well as for 

 grey, and a definite factor for vestigial wings as well as 

 for normal ones. In this way we are provided with 

 a recessive factor in the position required for inter- 

 change with the corresponding dominant factor when 

 crossing over takes place between the chromosomes. 



