228 HEREDITARY TRANSMISSION [CH. 



with that containing n and o. And lastly, if a break occurs 

 between M and N, it is unlikely that another will take place 

 between the contiguous factors N and 0, and correspondingly 

 MORGAN and his associates give evidence of "interference" 

 in crossing-over between two factors, in gametes in which 

 crossing-over has taken place between two factors placed 

 near them in the series. 



One considerable objection to this hypothesis must be 

 mentioned here, though further reference will be made to it 

 later. Crossing-over in Drosophila occurs only in the female ; 

 in the male characters that are normally coupled always 

 show absolute coupling. In the Silk-worm, on the contrary, 

 TANAKA has shown that there are several pairs of coupled 

 characters, and that, while crossing-over takes place be- 

 tween them in the male, in the female the coupling is abso- 

 lute. Although no cytological explanation of these facts is 

 as yet definitely known, there are indications in Drosophila 

 of differences between the prophases of the male and female 

 maturation stages which may account for them. A de- 

 scription of these will be given below (p. 235). 



Apart from the fact that the theory of MORGAN and his 

 colleagues with regard to the mechanism of coupling or 

 linkage and of crossing-over cannot be regarded as fully 

 established until the difference in behaviour in this respect 

 of the two sexes has been explained, the theory has also 

 been criticised on other grounds. The most serious of these 

 is that the facts are insufficient, as they are known at pre- 

 sent, to provide an adequate basis for the hypothesis. There 

 are undoubtedly a number of irregularities which involve 

 secondary assumptions for their explanation ; the ratios ob- 

 tained in breeding experiments are often unconformable, re- 

 quiring the assumption of differential mortality, of "lethal" 

 factors, of factors modifying the intensity of coupling, and 



