STUDIES ON CHROMOSOMES 103 



species (Oedancala, t. Montgomery), in others absent (Lygaeus). 

 In the Pyrrhocoridae (Pyrrhocoris, Largus) they are absent as 

 far as known. So characteristic is the behavior of these chromo- 

 somes as to leave not the least doubt of their essential identity 

 throughout the whole series; and this series may be regarded as a 

 progressive one, in one direction or the other, with the same reason 

 as incase of any other graded series of morphological characters. 

 The series thus shown in case of the ra-chromosomes is as gradual 

 and complete as in case of the Y-chromosome, and may with 

 the same degree of probability be regarded as a descending one. 

 Thirdly, it is probable that the chromosome-number may 

 change by sudden mutations that produce extensive redistribu- 

 tions of the chromatin-substance without involving any commen- 

 surate change in its essential content. Were gradual changes, 

 chromosome by chromosome, the usual mode of modification, 

 we should certainly expect to find such conditions as are seen in 

 Nezara, in Notonecta, or in the Coreidae, more frequently. In 

 some groups, however, we find wide differences of chromosome- 

 number between species even of the same genus, and even be- 

 tween those that are very nearly related, without any accompany- 

 ing evidence of a gradual process of transition for instance, 

 among the aphids and phylloxerans (Stevens, Morgan) or in the 

 heteropterous genera Banasa and Thyanta. (Wilson, '09d.) In 

 Banasa dimidiata the diploid number is 16 in both sexes, in the 

 nearly related B. calva 26. Of the two races of Thyanta custa- 

 tor described above, apparently identical in other visible char- 

 acters, one has in both sexes the diploid number 16, with a 

 simple X-chromosome, while in the other the diploid number of 

 the male is 27 and that of the female 28, and the X-chromosome 

 consists of two components. It is improbable that the dif- 

 ferentiation of these two forms has been accomplished by grad- 

 ual modifications, chromosome by chromosome. It seems far 

 more likely that the change took place by sudden mutation, invol- 

 ving a redistribution of the nuclear material which changed its 

 form but not in an appreciable degree its substance. In the well 

 known case of Oenothera gigas, derived by sudden mutation from 

 Oe. Lamarckiana, a change by sudden mutation is known to be 



