STUDIES ON 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 in 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 in 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 in 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 in any given case. 
It is now evident that the larger groups vary materially in respect 
to specific conditions. For instance, in 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 in 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 18, 1910. 
