VARIABLE PROTECTIVE RESEMBLANCE IN INSECTS 123 



develops from the eggs laid by a single butterfly. 

 Hence by keeping the companies separate, the varying 

 hereditary tendencies, due to different parentage, are 

 eliminated ; for a company of moderate size would 

 contain over one hundred larvae, and would therefore 

 furnish the material for several experiments. Con- 

 sidering also the abtuadance of the species, I determined 

 to employ it for the investigation of the process by 

 which the change of colour is effected. 



The pupae darker when crowded together 



Very early in the investigation a possible source 

 of error was detected. It seemed probable, when 

 many individuals were collected together at the sus- 

 ceptible period, which will be shown to occur towards 

 the end of larval life, that each of them would be 

 affected by that part of the surroundings which was 

 constituted by the black skins of its neighbours. It 

 was therefore necessary to take into account the 

 relative positions of the pupae, and, in the most 

 careful experiments, to place only a single individual 

 in each coloured case. Experiment soon showed 

 that these precautions were necessary. Many of the 

 darker pupae, shown in the table to be produced by 

 white and gilt surroundings, were proved to have 

 been influenced 'by mutual proximity, so that the 

 results would have been even more striking if this 

 source of error had been allowed for. 



