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. Altenburg's crosses of 

 Primula proved segregation of the linked factors to 

 occur after gonad formation. In flies Plough found 

 heat to affect Hnkage only if applied after the 

 completion of most, if not all, the gonial divisions 

 that might have "reduplicated" the eggs in question. 



At first it was doubted whether more than two 

 pairs of factors could show reduplication in the same 

 organism, but when it was experimentally proven 

 that two pairs were not the limit, the scheme was 

 extended. When gametic ratios not falhng into the 

 3, 7, 15 series w^ere found, the theory was modified 

 to permit other ratios. When it was found that the 

 result depended upon the way in which the factors 

 entered the cross, the ''i:)olarity" hypothesis was 

 added. Some further extension would be necessary to 

 account for interference. That interference is a wide- 

 spread phenomenon is shown by its occurrence in 

 Altenburg's Primula crosses, and in those of Ander- 

 son on corn — the only crosses outside of Drosophila 

 giving the exact relations of more than two factors. 



