xiv] HYBRIDS 215 



somes in gametogenesis is the mechanism by which the 

 segregation of Mendelian characters is brought about. These 

 difficulties will be discussed below, and it will be seen that 

 most of them are not insuperable, but first it may be advis- 

 able to mention two pieces of direct evidence in favour of 

 the hypothesis. 



FsDERLEYhas shown that when the moth Pygaera curtula 

 is crossed with P. anachoreta, the chromosomes behave in 

 the gametogenesis of the hybrid much in the same way as 

 in the cross, already referred to, between Biston hirtarius and 

 Nyssia zonaria. The reduced number in curtula is 29, in 

 anachoreta 30; the spermatogonia of the hybrid have 59, 

 but in the spermatocytes very few chromosomes pair; they 

 divide equationally in both divisions, with the result that 

 the mature germ-cells contain about 59. If, now, the hybrid 

 is mated back with pure anachoreta, a secondary hybrid with 

 nearly 89 chromosomes in its spermatogonia is produced. 

 In the spermatocytes of the secondary hybrid the two sets 

 of anachoreta chromosomes pair together, so that on the 

 spindle of the first spermatocytes there are about 30 large 

 (bivalent) chromosomes derived from anachoreta and about 

 29 univalents derived from curtula. The mature germ-cells 

 of the secondary hybrid therefore again contain nearly 59 

 chromosomes. Corresponding with these cytological facts 

 FEDERLEY found that in the primary hybrid some char- 

 acters of anachoreta and others of curtula were dominant, 

 while in other features the hybrid was intermediate between 

 the parents. When the hybrid was mated back with pure 

 anachoreta, no segregation of the moth characters took 

 place; all the secondary hybrids had the same characters 

 as the primary. In one larval feature, however, segregation 

 took place. Now since all the germ-cells of the secondary 

 hybrid contain both curtula and anachoreta chromosomes, 



