No. 615] THEORIES OF CROSSING OVER 261 



thing. The "variable force" theory would therein ap- 

 proach the chiasmatype theory, in which the diverse rela- 

 tions between the factors belonging to different pairs are 

 the primary, if not the exclusive, elements considered. 



As theories of other type become successively modified 

 so as to take into account the known facts: the fact thai 

 the chromosome actually is a linear aggregate; the fact 

 that the two chromosomes while in this linear condition 

 pair and intertwine ; the fact that cross-overs occur only 

 at the period when this occurs; the fact that two reces- 

 sive allelomorphs when mated do not produce normals, 

 while two recessives not allelomorphs do; the fact that 

 after two factors, A and B, are found to hold together in 

 one generation, if we mate their cross-overs A-b and 

 ar-B, we now find that it is A and b, not A and B that 

 tend to hold together (Bridges, 1917) ; the fact that when 

 a given factor is lost from a chromosome, others that 

 have low cross-over ratios with that factor are also lost 

 (Bridges, 1917a) ;— when the modifications required for 

 bringing these facts into relation with each other and 

 with others are introduced, it appears that the resulting 

 theory will come more and more to resemble the chiasma- 

 type theory. No theory is adequate that does not include 

 and bring into relation the facts just mentioned, for a 

 correct theory is nothing but a presentation of the facts 

 in their correct (verifiable) relations. 



PAPEES CITED 



