434 VARIETIES OF SPECIES OF INSECTS cuap. xxii. 



thickness of which scarcely exceeds three or four feet, and in 

 two quarries of very limited dimensions. The rare combina- 

 tion of causes which seems to have led to the faithful preser- 

 vation of so many treasures of a perishable nature in so small 

 an area, appear to have been the following: first, a river flow- 

 ing into a lake; secondly, storms of wind, by which leaves, 

 and sometimes the boughs of trees, were torn off, and floated 

 by the stream into the lake; thirdly, mephitic gases rising 

 from the lake, by which insects flying over its surface were 

 occasionally killed; and fourthl}-, a constant supply of car- 

 bonate of lime in solution from mineral 8j)rings, the calcareous 

 matter, when precipitated to the bottom, mingling with fine 

 mud, and thus forming the fossiliferous marls. 



Species of Insects in Britain and NoHh America, represented by 

 distinct Varieties. 

 If we compare the living British insects with those of the 

 American continent, we frequently find that even those 

 species which are considered to be identical are, neverthe- 

 less, varieties of the European types. I have noticed this 

 fact when speaking of the common English butterfly, Vanessa 

 atalanta, or "red admirable," which I saw flying about the 

 woods of Alabama in mid-winter. I was unable to detect 

 any difference myself, but all the American specimens which 

 I took to the British Museum were observed by Mr. Double- 

 da}^ to exhibit a slight peculiarity in the coloring of a 

 minute part of the anterior wing,* a character first detected 

 by Mr. T. F. Stephens, who has also discovered that similar 

 slight, but equally constant, variations, distinguish other lepi- 

 doptera now inhabiting the opposite side of the Atlantic, 

 insects which, nevertheless, he and Mr. Westwood and the 



* Lyell's Second Visit to the United States, vol. ii. p. 293. 



