244 Hybrids of Bistonine Moths 



paired in the spermatocyte divisions, so that these have only from 5 to 

 15 fewer than the somatic number. It is quite clear, however, that 

 some chromosomes, the number of which is small and probably some- 

 what variable, do find mates in the maturation divisions, and that there 

 are probably homologous chromosomes in the two species which pair 

 together, while the remainder are unable to do so. 



I do not propose to discuss at length the observations of which only 

 a condensed account has been given, firstly because to a considerable 

 extent they are similar to those of Federley, and he has discussed the 

 questions raised at considerable length; and secondly because I hope 

 next year to complete the investigation on additional material of the 

 same and other species, which should make a fuller understanding of 

 the problems possible. There are, however, a few points which seem to 

 require comment. 



The chief points of interest are (1) the difference in chromosome 

 number in nearly related species, and the behaviour of the chromosomes 

 in the hybrids, and (2) the possible causes of the fact that the cross 

 zonaria % x Mrtaria ^ gives only male offspring, while the converse 

 cross gives both sexes with excess of females. 



In the former group of phenomena we have important data on the 

 vexed question of the individuality of the chromosomes. Since there 

 are exactly four times as many chromosomes in zonaria as in hirtaria, 

 and since the zonaria chromosomes are so much smaller that the total 

 amount of chromatin, as judged by the size of the resting nuclei, is 

 approximately equal in the two species, it seems a fair inference to 

 suppose that the hirtaria chromosomes may be regarded as compound, 

 made up of units corresponding with the separate chromosomes of 

 zonaina. Since however there are large and small chromosomes in 

 hirtaria, and some in zonaria which are noticeably larger than the rest, 

 it is probable that not every Miliaria chromosome is made ujj of four 

 parts each of which corresponds with one zonaria chromosome, but that 

 the larger hirtaria chromosomes are made up of more than four units, 

 and the smallest ones are probably single. That is to say, if there are 

 56 units in the haploid group of zonaria, there are the same number of 

 units in hirtaria, but these are combined with one another in various 

 ways so that only 14 (or 13) chromosomes appear. For example, if the 

 56 haploid zonaria chromosomes are taken as units, the haploid set of 

 hirtaria might be made up of four chromosomes each composed of six 

 units, five of five, one of four, and three of one unit each. The total 56 

 is thus made up (4 x 6 + 5 x 5 + 1 x 4 + 3 x 1). This is of course not 



