120 ETHEL NICHOLSON BROWNE 



size, position and behavior, and also to the fact that the differ- 

 ences in tte chromosome number and grouping are relatively 

 slight in the different species, Notonecta offers an exceptionally 

 good field for such a comparative study. In order to ascertain 

 the relation between the chromosomes and somatic characters, 

 one of the chief aims of modern cytology, a comparative cyto- 

 logical investigation of closely related forms seems a method of 

 attack which should accompany the experimental method. 

 Moreover, it would seem that the study of different chromosome 

 groups within a genus should give a clue to the relationship and 

 possible evolution of the different species, so that a phylogenetic 

 tree based on the chromosomes would correspond with one based 

 on somatic characters and geographical distribution. A thorough 

 cytological examination has now been made, at least with regard 

 to the spermatogenesis, of six out of twenty species of Notonecta. 

 This comparatively limited study will, of course, not guarantee 

 any sweeping or final statements as to the relation of particular 

 chromosomes to particular body characters, or as to the precise 

 phylogeny of the species; only a few general and tentative sug- 

 gestions in this regard are offered in this paper. 



The problem concerning the chromosomes as giving a clue 

 to the relationship of different forms is one to which Montgom- 

 ery ('01, '06) devoted considerable attention. He argued that 

 the chromosomes are conservative elements and less modifiable 

 than the soma, and that they should therefore be considered as 

 of value in determining the relationship of a particular group. 

 He was, however, dealing with the Hemiptera in general and 

 was considering only the numerical relations of the chromosomes. 

 It is not surprising, therefore, that he found this problem in- 

 creasingly difficult of solution as his study became more exten- 

 sive. A glance at the chromosome numbers of different species 

 of Hemiptera as listed by him in 1906 will show how difficult 

 such a task would be. Had he limited himself to families, an 

 effort to find a rational explanation of a diversity in number of 

 chromosomes corresponding with the relationship of the genera, 

 would in many cases probably have proved fruitless. It has 

 been repeatedly shown by the work of Wilson, Boring, Stevens, 



