90 THE entomologist's record. 



33" F.); the colouring by a very moderate difference of temperature 

 during a later period, which, in the case of the Seknias and some others 

 he had localised as that which separated the inactive central period of 

 pupal life, from the very late period when the colouring of the perfect 

 insect had begun to show. In ii/usfraria, the natural difference of 

 markings, as well as of colouring, between the two seasonal forms, was 

 strong, and he had obtained a close approximation to either form, with 

 ease and regularity, by temperature applied to the pupa at the right 

 time. In ilhinaria and lunaria there was not so much difference in 

 markings, but there was, especially in the former, a very conspicuous 

 difference in colouring, and in both broods of both these species similar 

 results ensued, with practical invariableness. It was the same, but not 

 so regularly, with the single brooded E. mituuinm-ia. In V. urticce, 

 the results were in the same direction, but not so considerable, a 

 temperature of 50° to 60° appearing to develop the greatest intensity, 

 especially of the dark parts and the blue crescents, a relatively low tem- 

 perature (47°) causing a general increase of darkness, some specimens 

 approaching the northern var. polaris. In C. caja a moderately low 

 temperature caused the general ground colour of the forewings to be 

 darker, and that of the hind wings to be less yellow, and the dark 

 spots on them to spread, and increased in a striking degree the length 

 and breadth of the black abdominal bars. B. qiiercus and its var. 

 calluncE. showed slighter results in the same direction. Many more 

 experiments, and on many more species belonging to widely-separated 

 families among the Lepidnptera, should be tried before sufficient 

 materials could be accumulated for a satisfactory explanation of these 

 temperature effects ; but in the meantime he ventured to suggest that 

 some of them were due to the circumstance that a particular tempera- 

 ture was more suitable to health and vigour than any other was, and 

 tended consequently to produce larger size and greater intensity of 

 colouring and markings, but that this would only account for a small 

 part of the effects produced, especially in the seasonally dimorphic 

 species, and in those he had operated on there seemed to be what, in 

 the absence of further knowledge, might' be called a direct tendency in 

 a low temperature to cause darkness of markings. Mr. Merrifield 

 exhibited a large number of examples of the temperature effects pro- 

 duced. Mr. Fenn, after a few preliminary remarks, bearing testirnony 

 to the labour, patience and extreme care displayed in working out 

 these experiaients, for which great praise was due, and also to the 

 slight delicacy he felt in opposing Mr. Merrifield, after the trouble that 

 gentleman had taken in coming so far to read his paper before them, 

 proceeded to say that in a long experience of breeding Lepidoptera, 

 extending over more than thirty years, he had found that when treated 

 in a natural manner the proportion of abnormal varieties bred to be 

 under one per cent. He divided variation into two sections, viz.^ natural 

 and unnatural or artificial. With regard to the former, that m.ight 

 again be divided into three nearly equal causes, heredity, moisture 

 and natural selection ; he gave the reasons for each, with examples. 

 Respecting the latter, Mr. Fenn held that an examination of most of 

 our extraordinary varieties would show that the variation in question 

 was caused by some form of disease, some weakening of the vitality 

 or constitution of the insect, either in the larva or pupa state, not of 



