434 VARIETIES OF SPECIES OP INSECTS CHAP. xxn. 



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

 two quarries of very limited dimensions. The rare combination 

 of causes which seems to have led to the faithful preservation 

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

 area, appear to have been the following : first, a river flowing 

 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 occasion 

 ally killed : and fourthly, a constant supply of carbonate of lime 

 in solution from mineral springs, the calcareous matter, when 

 precipitated to the bottom, mingling with fine mud, and thus 

 forming the fossiliferous marls. 



Species of Insects in Britain and North America, repre 

 sented 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 6 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- 

 day to exhibit a slight peculiarity in the colouring 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 sides of the Atlantic, 

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



* LyelTs_Second Visit to the United States, vol. ii. p. 293. 



