LINKAGE 77 



ratios which do fall into it are no more frequent than 

 would be expected from a chance distribution. 



Another assumption upon which the reduplication 

 hypothesis is based is the old idea of somatic (pre- 

 reductional) segregation. This hypothesis, once ad- 

 vocated by Roux and Weismann as an explanation of 

 differentiation, is opposed by a large body of experi- 

 mental evidence from the fields of regeneration and 

 experimental embryology, and has been given up by 

 practically all students of developmental mechanics, 

 including Roux himself. 



At first it was doubted whether more than two pairs 

 of factors could show reduplication in the same organ- 

 ism, but when it was experimentally proven that tw^o 

 pairs were not the limit, the scheme w^as extended. 

 When gametic ratios not falling into the 3, 7, 15, 

 series were found, the theory was modified to permit 

 other ratios. When it w^as found that the result 

 depended upon the way in which the factors entered 

 the cross, the ^'polarity'' hypothesis was added. 

 Some further extension will be necessary to account 

 for the phenomenon of '^interference" discussed 

 above. 



