/2 ETHEL NICHOLSON BROWNE 



between the eyes. It may be that the wing color is directly" cor- 

 related with the fusion and separation of the two chromosomes, 

 and that the other somatic differences are connected with other 

 chromosomes, which, as I have stated, differ in size in the differ- 

 ent species. 



In the Acrididae, McClung ('05) has found that a particular 

 genus, Hesperotettix, is distinguished from others of the family 

 by a special arrangement of the chromosomes, by which the acces- 

 sory is always associated with another chromosome forming a 

 multiple element. He concludes that this arrangement "is 

 genetically conneeted with the subsequently appearing char- 

 acters" (op. cit. p. 326). This correlation of a multiple chromo- 

 some element and a generic difference in the Acrididae is directly 

 comparable to the correlation of a multiple chromosome element 

 and a specific difference in Notonecta. 



The correlation of a definite number of chromosomes with a 

 particular species is a well established fact throughout the animal 

 and plant kingdoms, and is admitted by practically all cytologists, 

 with only a few exceptions. In several cases, however, it has 

 been shown that the number is constant for the individual, but 

 differs for different individuals. This is sometimes due to the 

 presence of 'supernumerary' chromosomes, as in ]Metapodius, 

 Banasa calva, Diabrotica and Ceuthophilus (Wilson '09 a, Stevens 

 '12 a, b), and sometimes to the fact that two types of chromo- 

 some groups occur within the species, one with twice the number 

 of the other, as in the well known cases of Ascaris megalocephala, 

 Echinus microtuberculatus and Helix pomatia, and as in Artemia 

 salina, as recently pointed out by Artom ('11). Cyclops viridis 

 is apparently a species in which different numbers occur in differ- 

 ent varieties (Chambers '12). With these and possibly a few 

 other exceptions, the number of chromosomes is a specific char- 

 acteristic, although occasional fluctuations may occur. 



Further, there are many cases where closely related species 

 have the same number of chromosomes. For example, five spe- 

 cies of Euschistus have the same number (Montgomery, Wilson) ; 

 four species of Sagitta (Stevens '10), and three species of Ceresa 

 (Boring '07). In other cases, related species differ only slightly 



