Poulton, Mimicry and Natural Selection. 
(oS) 
locality, why should they more than all other Rhopalocera arrive 
independently at the same evolutionary stage as regards visible 
characters, why should sexual selection operate so exclusively 
upon them in the direction of producing a common likeness? 
None of these questions can be answered. The facts remain mere 
coincidences under all theories except Natural Selection. In other 
words Natural Selection is the only satisfactory interpretation. 
Mimicry among Rhopalocera is much less common as we 
pass into northern regions, but there is one excellent example in 
temperate North America which serves to shew how superficial an 
interpretation is that offered by the theory of External Causes and 
how completely it breaks down when examined with a little care. 
With comparatively few exceptions the insect fauna of the Nearctic 
Region is that of the great northern circumpolar land-belt. These 
exceptions are intruders from the tropical South, and among them 
is the large Danaine butterfly Anosia plexippus which now 
ranges over the United States and a large part of Canada. In 
tropical America closely similar representative species, sub-species 
or forms still persist. This abundant Danaine butterfly affords 
the model which is closely resembled by an indigenous Nymphaline 
butterfly which we should place in the genus Limenitis, although 
some American naturalists prefer to put the Nearctic species in a 
separate genus, Basilarchia. There are also other mimics 
among the species of the North American Limenitis (Basilar- 
chia), but two of them are non-mimetic and enable us to recon- 
struct the appearance of their close ally before the intrusion of 
the great Danaine model. In the New World the genus Lime- 
nitis is confined to the Nearctic Region with the exception of a 
single species, a form of the mimetic L. astyanax (Fabr.), which 
just enters the borders of Mexico. If butterfly colours and patterns 
are the expression of the direct influences of the environment, then 
it is clear that the indigenous non-mimetic species of Limenitis 
(Basilarchia) are an expression of Nearctic conditions, and accor- 
ding to the theory of External Causes, the invader from the South 
should have come to resemble them instead of drawing an ancient 
Nearctic species far away from the ancestral colours and patterns 
into a close superficial likeness to itself. The fact that certain 
species of a single genus should thus be entirely mimetic while 
others are entirely non-mimetic and preserve the ancestral 
appearance, has been sometimes urged, for example by late 
Professor J. O. Westwood, against the interpretation afforded 
by the theory of Natural Selection. It is a real difficulty to 
the theories of external and internal causes; for, as regards the 
first, we should expect the closely related species of a genus to 
react similarly to the local conditions rather than that certain of 
them should react quite differently from the others but quite 
similarly to the species of distantly related sub-families; and, as 
