C. E. MCCLUNG. 



characters that are remarkably constant. It is, in fact, sometimes 

 difficult to establish characters of sufficient importance to differ- 

 entiate subfamilies by their presence or absence. This precision 

 and fixity of bodily organization is accompanied by a correspond- 

 ing stability of the chromosome complex, which indicates that 

 throughout the family there is a correspondence between indi- 

 vidual chromosomes and their products in development. By 

 this I mean that if we can fix upon any particular chromosome 

 by some peculiarity of structure or behavior and associate it with 

 definite body characters we will find that same chromosome in all 

 the species of the family governing the development of the same 

 somatic structures. Individual chromosomes have therefore a con- 

 tinuity of descent in the same way that cells and organisms have. 

 Comparing the Acrididae with the Locustidae it appears that in 

 the spermatogonia there is a difference of ten chromosomes in 

 favor of the latter group. While these two families are well 

 separated it is due more to differences of common characters than 

 to the absence in one of characters possessed by the other. We 

 must conclude, therefore, that the chromatin governing the 

 development of these common characters is present in both 

 families. It is evident, however, from the enumeration of the 

 chromosomes, that it is differently associated in the two groups 

 with the exception of that contained in the accessory chromo- 

 some. We cannot for this reason compare chromosomes with 

 chromosomes throughout the whole complex of the two families 

 and it is plain that the differences in structure are due to the dif- 

 ferences in composition and association of the chromosomes. 

 This is foreshadowed, or indicated, by the differences of associa- 

 tion that prevail between genera, as is suggested by the appear- 

 ance of multiple chromosomes. These associations may further 

 point the way to an understanding of the differences in number 

 between different families, the smaller one being brought about 

 by the parmanent union of independent chromosomes, rather 

 than by the entire loss of elements. 



5. Chromosomes in Variation. 



A discussion of the relation of chromosomes to one another in 

 different groups leads to an enquiry into the relation of characters 



