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unlike each other, are also undistinguishable in c? imago, the 

 importance of carefully studying the larva state of every insect 

 becomes at once apparent. Have these two pairs of species 

 been apparently identical in the imago, either cT ? or d* only, 

 and actually distinct in the larva, for all time since their supposed 

 original creation ? or were they identical both in larva and imago, 

 perhaps some millions of years ago, and did an individual Halesidota 

 then acquire a taste for buttonwood leaves instead of oak and bass- 

 wood leaves, and an individual Dryocampa acquire a taste for honey 

 locust leaves instead of oak leaves, and propagate the same taste in 

 its descendants through all time ? Is it possible that in the course of 

 millions of years a permanent change in food should have then pro- 

 duced external structural differences in the larva such as those we see 

 between the larvae of H. tessellaris and Antipliola, and between the 

 larvfe of S. distigma and D. bicolor, and internal sti'uctural differences 

 in the imago such as would prevent the sexual intermixture of the 

 two races ? I am acquainted with some cases where change of food 

 produces a constant and very remarkable change in coloration in the 

 larva, though none of any consequence in the imago. One of these 

 cases, which is fortified by the authority of Abbott, (^Datana ministra) 

 I have recorded in the Proc. Ent. Soc. of Philadelphia (Vol. i. p. 

 296) ; and Dr. Fitch has recorded the very instructive fact that the 

 mere shifting of their quarters, from the leaf to the ear of one and 

 the same plant, produces in the descendants of Aphis avence (the 

 grain plant-louse) a constant change of color from green to yellow or 

 reddish-yellow. Again, that even structural differences may be prop- 

 agated by hereditary descent in Mollusca to a whole local race, we 

 learn on the authority of Dr. P. P. Carpenter. " It is a curious fact," 

 says that author, " that whatever be the form of the operculum in the 

 different tribes of predaceous mollusks, whenever it has been broken, 

 and has to be repaired by the animal, it always takes a simple oval 

 shape, with concentric layers, the nucleus being in the middle. In 

 one place on the English coast there is found a race of Buccinum un- 

 datum (the common whelk of the English and American coasts), which 

 perpetuates a very abnormal condition. They have two small oper- 

 cula of more or less irregular shapes, but each of concentric elements. 

 Probably their remote ancestor met with an accident, and has trans- 

 mitted her mode of repairing the fracture to her descendants." (Lec- 

 tures on Mollusca, Smithson. Rep. 1860, p. 176.) The phenomena of 

 what are known as " self-tailed dogs" are familiar to many Englishmen. 

 In the great grazing counties in the north of England a peculiar 

 breed of dogs, known as the " Cur-dog," has been used for ages for 

 driving cattle, as the Colley-dog is used for driving sheep, and so com- 

 mon has been the practice of cropping their taUs that Bewick actually 



