74 THE CHROMOSOMES 



which are probably to be interpreted as multiple 

 gene -differences.^*^ Hybrids can be obtained be- 

 tween the two species, but they are sterile. 



Inversions of the type just described are probably 

 among the commonest cytological differences between 

 closely related species (they occur also inside single 

 species 37, 46j jj^ addition to these and single gene- 

 diflferences, however, the parent species of a hybrid 

 may differ in respect of (1) chromosome number, 

 (2) chromosome size, and (3) rate and extent of con- 

 traction of the chromosomes at mitosis and meiosis. 

 Where the chromosome numbers of the two parent 

 species differ one of the two haploid sets in the 

 hybrid will contain more chromosomes than the 

 other and the ' extra ' chromosomes will necessarily 

 be unable to pair at zygotene and will form univalents 

 at the first meiotic division. As far as the others are 

 concerned, there is a possibility of them all pairing 

 and forming bivalent s, but where the chromosome 

 numbers of the two species differ the degree of 

 homology between the two haploid sets is usually 

 so incomplete that pairing and chiasma-formation 

 only take place in a few chromosomes ; the number 

 of univalents is thus usually considerably in excess 

 of the number of ' extra ' chromosomes in such cases. 

 For example, in hybrids between the moths Satumia 

 pavonia and S. pyri (haploid numbers 29 and 30 

 respectively) only about 5 to 6 bivalents are formed 

 as a rule, the remaining 47 or 49 chromosomes being 

 left as univalents.^** 



Where the chromosome numbers of the two parent 

 species are the same all the chromosomes may pair 

 but do not necessarily do so. Thus in the hybrids 

 between the moths Celerio euphorbiae and C. galii 

 (both with a haploid number of 29) each euphorbiae 

 chromosome pairs with a galii one so that 29 bivalents 

 and no univalents are formed. On the other hand, 

 in the hybrid between Pergesa elpenor and Celerio 

 euphorbias (each with a haploid number of 29 as 



