CHROMOSOME STUDIES 269 



ficity between members of the same pairs. The same may be 

 said of the no. 4's in the deficient no. 4 tetrad. 



And now it seems to me that in the foregoing data on con- 

 stancy in number, size and behavior, and in the evidence for 

 genetic — and in some cases individual — continuity of the chromo- 

 somes, we have, especially in the Tettigidae, proof of the pres- 

 ence in the germ cells of a structure sufficiently stable and con- 

 tinuous to furnish a physical basis for heredity. Again, this 

 structure, while sufficiently stable to account for heredity, the 

 relationship of species, genera, etc., has evidently been plastic 

 enough to allow of variation in the past sufficient to account 

 for what we find characteristic of at least two subfamilies of the 

 Tettigidae, the Batrachidinae and Tettiginae, and of the genera 

 within these, when compared with each other, and of the respec- 

 tive species of the genera, in so far as we are able to see. 



The family Tettigidae itself in its thirteen-chromosome 

 trait is distinctly and clearly marked off from the subfamilies 

 Truxalinae, Oedipodinae and Acridinae of the Acrididae in their 

 twenty-three chromosome trait. This degree of difference, 

 as I have already pointed out, is paralleled in the somatic char- 

 acters, both internal and external. The degree of difference in 

 relative length of chromosomes between the genus Tettigidea, 

 of the subfamily Batrachidinae, and the two genera Paratettix 

 and Acridium of the Tettiginae is greater than is the difference 

 between the genera Paratettix and Acridium themselves. This 

 may be seen in the tables (I-XX). Among the species of Acrid- 

 ium the same phenomenon may be seen, though to a much 

 less degree. As we descend to the species, the differences be- 

 tween nearly related ones are so little that it is practically 

 impossible to detect them. 



The variations I have described are found existing today, 

 pernianent for a number of organisms which systematists have 

 grouped into families, subfamilies, genera, and in some cases 

 species. Whether any one of these three genera of the family 

 Tettigidae represents the more ancestral genus of the family, I 

 am not ready to say. This may be determined later. Suffice it 

 to say that from the evidence displayed in the series of chromo- 



