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 linkage 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 falling into the 
3, 7, 15 series were 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 ‘polarity’? 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. 
