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on their own limits. Hence, if such be the case, as I contend that it 
usually is (the exception to the rule being the result of accident, and 
by no means a part of the original design), it may perhaps be a 
problem how far we are justified in rejecting many large and natural 
assemblages, through the fact that they blend both at their com- 
mencement and termination imperceptibly with others, their precise 
boundaries being dimly defined.” It would appear that we are not 
justified in rejecting any “ natural assemblage,” but the distinctions 
between the extremes of two adjacent assemblages should be defined 
as precisely as possible. 
One other point presents itself to me, viz. that groups which appear 
to be abruptly and distinctly isolated when our knowledge is imper- 
fect, or based on the fauna of a very small area, are very frequently 
found to be closely united with others when our knowledge becomes 
more complete, and when intermediate forms from other districts are 
discovered, so that their separation and isolation are in such cases 
directly the result of ignorance rather than an actual fact. 
The question of convenience appears to me at best an unsatis- 
factory one. Personally I am inclined to reject it altogether ; but 
when convenience is urged by naturalists, it should, it appears to me, 
always be with the proviso that it breaks no natural law. 
To illustrate this point we may turn to the British Vanessids. 
Scudder has long since pointed out that all our British species are 
practically representatives of distinct evolutionary groups, and has 
characterised them in the egg, larval, pupal, and imaginal stages as 
distinct genera. It is further well known that if the Vanessids of the 
world be considered, our British species will fall into various genera 
belonging to the tribe Vanesstd?. For example, our comma 
butterfly (cadbum) is a member of the great American genus 
Polygonta, consisting of a large number of closely allied species, all 
with the distinct characters of our single British species. Our 
British cavduz and azalanta fall into the genus Pyrameis, of which 
there are many species both in Asia and America. The large 
tortoiseshell falls into an entirely different genus—/ugonéa—from 
that of the small tortoiseshell butterfly—Ag/azs, both of which have 
Asiatic and American representatives. Yet a recent author, writing 
on these from a purely British outlook, says, “The genus Vanessa 
is now broken up into several sub-genera for the purpose of 
classifying the numerous exotic species, but for the few that we 
possess this subdivision seems unnecessary.” Surely this was written 
in pure ignorance of the facts. From 1816 onwards the Vanessids 
have been properly considered as a tribe, containing in_ itself 
many well-defined genera, and the advanced writers of every decade 
have recognised the fact. The first part of the quotation, too, is 
incorrect, for Hubner never considered the subdivisions as anything 
lower than of generic value, and Scudder distinctly diagnoses them as 
such, and at present I must confess that I am not aware of what 
this writer means by a sub-genus. 
