CHANGES IN PROTECTIVE COLORATION 89 



Having deposited her eggs, she climbs up to the stigma, the 

 natural entrance to the interior of the ovary, into which 

 she presses the ball of pollen, thus sealing it up. Of course 

 this also fertilises the ovules, and, moreover, as she always 

 lays her eggs in a different flower to that from which she 

 collected the pollen, cross-fertilisation is secured. The moth 

 only lays a few eggs in each ovary, which hatch out into larva;, 

 which feed upon the fertilised ovules of the Yucca. As, 

 however, the ovules of the plant are very numerous, there 

 are plenty, both to provide food for the larvse of the moth 

 and also for the reproduction of the plant. 1 



It has been recorded that, during recent years, many 

 species of moths found in Lancashire and Yorkshire, and 

 thence over a gradually extending area, have been growing 

 much darker. This is probably due to the darkening of 

 their environment, brought about by smoke and the dying 

 off of the lichen. The process has been gradual, and in 

 every case except one intermediate gradations between the 

 dark and light forms are found, the lighter becoming rarer, 

 the darker commoner. The one exception is Ampkidasys 

 betularia. A dark variety, doubledayaria, was known to 

 exist before the present darkening of the environment, but 

 was rare. At the present time the dark variety has sup- 

 planted the light in these districts. 2 This might be claimed 

 as a mutation, but it is explicable in another way. The dark 

 variety already existed, and there is nothing to show that 

 it was not produced locally in a very gradual manner. 

 When the general environment over a large area changed, 

 the dark variety had a great advantage over the light, and 

 it would most likely happen that the already existing dark 

 race would multiply more rapidly than the selection of 

 small variations would modify the light race. On the 

 other hand, we have, in some cases, an obviously gradual 

 change produced by the selection of small variations. It 



1 Riley, Third Ann. Rep. Missouri Bot. Gdn., 1892. 



' Porritt, G. T., 'British Association Report, p. 316, 1906 ; Tutt, J. W., 

 Entomologist's Record, vol. i., 1890-91 ; Poultou, op. cit., pp. 308-10. 



