VARIATION AND ABERRATION IN BUTTERFLIES. 71 



imaginal stages ; summary of conclusions, especially as to seasonal 

 double-brooded species. 



1890, Dec. 3rd (Trans. 1891, p. 155). — S. tetralunaria, E. mitum- 

 naria, Vanessa urticcB ; forcing and cooling, and different eifects caused 

 by temperature according to period of pupal stage in which applied ; 

 negative results from moisture applied in pupal stages of S. tetralunaria 

 and E. autuvuiaria ; possible use of temperature experiments on pupae 

 in tracing evolution of markings on wings. 



1891, Dec. 2nd (Trans. 1892, p. 33).— S'. lunaria, S. bilunaria, and 

 S. tetralunaria, Plati/pteri/x falcataria , Vanessa urtictB, Bomhyx quercus 

 and var. calluna;, Chelonia caia ; negative results with spring emerg- 

 ence of P. machaon and P. podaliriiis, Thais polyxena, and some other 

 species ; negative results as to effect of light on pupae of S. tetralunaria 

 and Bomhyx cynthia. 



1892, Dec. 7th (Trans. 1898, p. 55). — Pieris napi, Vanessa atalanta, 

 Chrysophamis phlceas, Zonosonta [Ephyra] pxinctaria. [1893, Feb. 22nd, 

 Trans, p. 69, observations by Dr. F. A. Dixey on the phylogenetic 

 significance of the variations produced by difference of temperature on 

 Vanessa atalanta.^ 



1894, March 14th (Trans. 1894, p. 425).— Piern napi, Pararge 

 egeria, Cidaria silaceata, Araclmia levana, Vanessa polychloros, V. ata- 

 lanta, V. c-album, V. io, and V. antiopa, with observations (p. 439) by 

 Dr. F. A. Dixey, on the results obtained.] 



I have already treated of the present subject, though by a 

 different title, in the * Manual for Collectors of the European 

 Lepidoptera,' 1891, pp. 107-128. My starting-point was there 

 the divergent forms — varieties and aberrations — considered in 

 themselves ; but in the course of the examination of these forms 

 the causes were indicated by means of which these divergences 

 may perhaps have arisen. This was the only method at that 

 time open, since but few positive statements could then, unfortu- 

 nately, be made as to the causes of the phenomena in question. 

 It was almost entirely a case of hypotheses with a greater or less 

 degree of probability. 



We will here adopt the converse method ; that is to say, we 

 will devote our chief attention to the causes which have brought 

 about these phenomena ; for I am now, in consequence of careful 

 investigations extending over many years, in a position to 

 contribute in a more practical manner to the solution of this 

 question. 



There are a very large number of external conditions which 

 have a decided influence on the organic world. Among these 

 are — degrees of temperature and of humidity, amount of light, 

 barometric pressure ; I might further mention the chemical 

 influence of food-stuffs and possibly electricity. I shall here 

 confine myself in the main to those classes of factors whose 

 operation I have myself tested (these being, it will be under- 



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