60 



flies in the dark. Mr. McArthur says that in Unst, the 

 most northern of the Shetland Isles, more or less typical 

 H. Immtdi occur, as well as the other form ('' Moths of the 

 British Isles," vol. ii, p. 361). This var. thnleiis also occurs as 

 an aberration in Holland. There seems to be no reason to 

 doubt that the ovum, larva and pupa of both forms are 

 identical, and that the type and var. thdeiis would freely 

 interbreed, but unfortunately I have no proof of this. 



There is one most beautiful example of what I call pure 

 imaginal evolution, and though I am personally only 

 acquainted with the imago, the early stages have been 

 described and its affinities pointed out by one of the greatest 

 entomologists of our time. The eggs and their disposition 

 in imbricated sets, the larva and its habits, and the structure 

 of the pupa, all show distinctly that Arsilonchevenosa belongs 

 to the Viminia group of the AcronyctidcB, and is very closely 

 allied to Acronyda rumicis. The coloration of the imago is, 

 however, absolutely different. From its long abode among 

 the reeds in the fens it has developed the protective colour 

 and markings which we so often find associated with the 

 reed-frequenting Lepidoptera (" Ent. Record," vol. i, p. i, 

 et seq.). 



The forces which bring about the evolution of an organism 

 appear to be of two distinct kinds : The outside forces, 

 which, as the outer conditions of life change, call upon the 

 organism to alter and meet the changes, and the inner forces, 

 which, while holding the organism within certain limits, 

 allow it to respond more or less perfectly, sooner or later, to 

 the demands made on it by the outside forces. The Lepi- 

 doptera, I think, offer some very good illustrations of this 

 theory, showing how similar demands made by the outside 

 forces have been met in different ways by different species. 

 A great many pupae have developed hooks or spines on the 

 dorsum, which, in various ways, aid the imagines to free them- 

 selves from the cocoon and from the pupa-shell. Dr. Chap- 

 man, speaking of the dorsal hooks on the pupa of the genus 

 Tischeria, remarks that one species has " small spines uni- 

 formly over the segment." In another species these are 

 " segregated into a central patch." In a third species they are 

 " massing towards the anterior margin " ; whilst in a fourth 

 they are " beginning to form a posterior row " ("Brit. Lep.," 

 vol. ii, p. 97). This shows how species of that genus have 

 met, up till the present time, the common demand for dorsal 

 hooks. The environment of the larvae of those two very 

 closely allied species, A crony eta tridens and A. t>si, appears 



