T24 YORKSHIRE NATURALISTS' UNION. 



might be, in which they were thus placed at a disadvantage, 

 would have its effect in diminishing their numbers, promoting 

 the survival of only the fittest forms. If this is so it is 

 sufi&ciently obvious that the first males on the wing have the 

 best chance of transmitting their colour by an hereditary 

 process to the succeeding generation ; and if these males were 

 always or usually the darkest of the brood, their progeny would 

 also be for the most part dark. 



It may be objected that Professor Tyndall's experiment * 

 with a white substance specially chosen on account of its 

 perviousness to visible rays, and its extreme imperviousness 

 to invisible ones, absorbing heat more rapidly than a dark 

 substance specially chosen on account of its opposite ten- 

 dencies, proves, as he says, that ' conclusions as to the 

 influence of colour may be altogether delusive,' and that it by 

 no means follows that because a piece of black cloth placed 

 upon snow will sink below the surface, whilst a piece of white 

 cloth in a similar position remains above it, therefore a black 

 insect and a white insect would behave in the same manner 

 under similar conditions. To set this doubt at rest I took 

 advantage of one of the few sunny days during the last fall of 

 snow, with a view to test the comparative rapidity of heat- 

 absorption in some of our common Lepidoptera. On the 23rd 

 of January, at 11.30 a.m., I placed two specimens of Tatiagra 

 chcerophyllata, a black insect from the Yorkshire moorlands, 

 and three of Acidalia imvmtata, a white insect from the 

 Norfolk fens, on a smooth surface of snow exposed to bright 

 sunshine at an angle of about 45°, with them I put a male of 

 Colias edusa, a pair of Satyriis tithoims, a pair of Thecla 

 querciis, and three specimens of Lithosia strami7ieola. A 

 thermometer lying on the grass by the side of the snow stood at 

 48° Fahr. At noon Chcerophyllata already showed decided 

 signs of melting the snow, so did Satynis and the female Thecla^ 



* ' Fragments of Science,' vol. i, p. 88. 



Trans. Y.N. U., 1883 (pub. 1885). Series D 



