130 THEORY OF EVOLUTION 



the two pairs of characters enter, they tend to 

 remain in that combination. 



If one admits that the sex chromosomes carry 

 these factors for the sex-linked characters 

 and the evidence is certainly very strong in 

 favor of this view it follows necessarily from 

 these facts that at some time in their history 

 there has been an interchange between the two 

 sex chromosomes in the female. 



There are several stages in the conjugation 

 of the chromosomes at which such an inter- 

 change between the members of a pair might 

 occur. There is further a small amount of 

 direct evidence, unfortunately very meagre at 

 present, showing that an interchange does 

 actually occur. 



At the ripening period of the germ cell the 

 members of each pair of chromosomes come to- 

 gether (fig. 49, e). In several forms they 

 have been described as meeting at one end and 

 then progressively coming to lie side by side as 

 shown in fig. 63, e, f, g, h, i. At the end of 

 the process they appear to have completely 

 united along their length (fig. 63, j, k, 1). It 

 is always a maternal and a paternal chromo- 



