1896.] . Entomology. 681 
no longer exist; the single adaptive one would have replaced it. But 
it is obvious that the appearance of complicated details of marking and 
color, such as ocelli are, cannot be simply the direct effect of heat or 
cold, drought or humidity. These influences are not the actual causes 
of such formations, but only the stimulus, which sets their primary con- 
stituents free, i. e., induces their development, as I tried to demonstrate 
in the lecture above noted. As the sufficient cause of the sleep of the 
marmots does not lie in the cold, but in the organization of the animal 
which is adapted to the cold, and as the cold only brings the existing 
predisposition to winter sleep into play, so among these butterflies with 
adaptive seasonal dimorphism the display of the one or the other mark- 
ing is apparently connected, partially, at least, with one of the above 
named outward influences, although in reference to these trophical 
butterflies we do not yet know to which of them. 
We recognize temperature as the stimulus to development with the 
cases of seasonal dimorphism of our indigenous butterflies, as in all 
cases of seasonal dimorphism, which have hitherto proved experimen- 
tally, it is always high and low temperature which gives the outward 
impulse to the appearance of the one or the other form where this 
impulse did not come exclusively from within. 
There are, therefore, two different sources of the appearance of sea- 
sonal dimorphism: on the one hand, the direct action of alternating 
external influences, viz.: temperature, can bring about this change in 
the outward appearance; and on the other hand, the processes of selec-. 
tion. It is therefore necessary to consider these two kinds of seasonal 
dimorphism separately. It will certainly not always be easy to decide 
between them when a particular case has to be dealt with, as at present 
it is not always possible to say whether a coloring or marking has a 
definite biological value or not. Both causes also may co-operate in 
in one species. 
Note on the Classification of Diplopoda.—The admitted im- 
possibility of formulating a generally satisfactory definition of the term 
species exists partly because systematists have used it in the greatest 
variety of applications, and partly because natural groups are so 
diverse in structure and developmental history that a scheme calcu- 
lated to elucidate one may increase confusion in another. It is hence 
desirable in proposing or making use of a classification to recognize as 
clearly as possible the conceptions under which the arrangement into 
the various categories of natural groups has been made. 
The structure and distribution of the Diplopoda make it advanta- 
geous and usually easy to arrange them into species, which are groups 
