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 Glaueus, inhabits the 

 same region ; and though, down to the time when 

 Boisduval published his " Species General," 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 



