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ILLUSTRATIVE OF NATURAL SELECTION, 153 
supposed to exhibit one of the intermediate steps in 
that process, which has been accidentally preserved in 
company with its more favoured rivals, though its 
extreme rarity (only one specimen having been seen 
to many hundreds of the other form) would indicate 
that it may soon become extinct. 
The only other case of polymorphism in the genus 
Papilio, at all equal in interest to those I have now 
brought forward, occurs in America; and we have, 
fortunately, accurate information about it. Papilio 
Turnus is common over almost the whole of tem- 
perate North America; and the female resembles the 
male very closely. A totally different-looking insect 
both in form and colour, Papilio Glaucus, inhabits the 
same region; and though, down to the time when 
Boisduval ‘published his ‘ Species Général,” no con- 
nexion was supposed to exist between the two species, 
it is now well ascertained that P. Glaucus is a second 
female form of P. Turnus. In the ‘Proceedings 
of the Entomological Society of Philadelphia,” Jan., 
1863, Mr. Walsh gives a very interesting account of 
the distribution of this species. He tells us that in 
the New England States and in New York all the 
females are yellow, while in Illinois and further south 
all are black; in the intermediate region both black . 
and yellow females occur in varying proportions. 
Lat. 37° is approximately the southern limit of the 
yellow form, and 42° the northern limit of the black 
form; and, to render the proof complete, both black 
and yellow insects have been bred from a single batch 
