STUDIES OX CHROMOSOMES 107 



individual fluctuations occasionally are seen in the number of the 

 chromosomes, in the process of synapsis, in the distribution of the 

 daughter-chromosomes, and hi all other cytological phenomena. 

 It is, however, also true that most observers who have made pro- 

 longed, detailed and comparative studies of any particular group, 

 have sooner or later reached the conviction that hi each species 

 all the essential relations in the distribution of the chromosomes 

 conform with wonderful fidelity to the specific type. So true is 

 this that the species may often at once be identified by an expe- 

 rienced observer from a single chromosome-group at any stage of 

 the maturation-process. No one, I believe, who has engaged for 

 a series of years in the detailed study of such a group, for instance, 

 as the Hemiptera or the Orthoptera, returning again and again to 

 the scrutiny of the same material, can be shaken hi the convic- 

 tion that the distribution of the chromosomes follows a perfectly 

 definite order, even though disturbances of that order now and 

 then occur. But it is equally important to recognize the fact 

 that this order has undergone many definite modifications of 

 detail from species to species, and that while all cases exhibit cer- 

 tain fundamental common features, we cannot without actual 

 observation predict the particular conditions hi any given case. 

 It is now evident that the larger groups vary materially in respect 

 to specific conditions. For instance, hi the orthopteran family of 

 Acrididae (McClung) the relations seem to be far more 

 uniform than such a group as the Hemiptera, where great spe- 

 cific diversity is exhibited, the details often changing from species 

 to species hi a surprising manner witness the species of Aphis 

 or Phylloxera (Stevens, Morgan), those of Acholla (Payne) or 

 of Thyanta (Wilson). In these respects, too, the cytologist finds 

 his experience running parallel to that of the experimenter on 

 heredity; and here, once more, we find it difficult not to believe 

 that both are studying, from different sides, essentially the same 

 problem. 



December 13, 1910. 



