Influence of Environment on Animals 215 



This is a comparatively simple case. Much more compli- 

 cated conditions were fomid l)y Keeble and (lam])l(* in other 

 crustaceans, e.g., in Hippolyte cranchii, but the iiifluciiee of the 

 surroundings upon the coloration of tliis form was also satis- 

 factorily analyzed by these authors. 



While many animals show transitory changes in color under 

 the influence of their surroundings, in a few cases permanent 

 changes can be produced. The best examples of this are those 

 which were observed by Poulton in the chrysalids of various 

 butterflies, especially the small tortoise-shell. These exjKTi- 

 ments are so well known that a short reference to them will 

 suffice. Poulton^ found that in gilt or white surroundings the 

 pupae became light colored and there was often an immense 

 development of the golden spots, ''so that in many cases the 

 whole surface of the pupae glittered with an apparent metallic 

 luster. So remarkable was the appearance that a physicist, 

 to whom I showed the chrysalids, suggested that I had played a 

 trick and had covered them with goldleaf." When black sur- 

 roundings were used, ''the pupae were as a rule extremely dark, 

 with only the smallest trace, and often no trace at all, of the 

 golden spots which are so conspicuous in the lighter form." 

 The susceptibility of the animal to this influence of its surround- 

 ings was found to be greatest during a definite period when the 

 caterpillar undergoes the metamorphosis into the chrysalis stage. 

 As far as the writer is aware, no physico-chemical explanation, 

 except possibly Wieners' suggestion of color photogra])hy by 

 mechanical color adaptation, has ever l)een offered for the 

 results of the type of those observed by Poulton. 



V. EFFECTS OF GRAVITATION 



a) Experiments on the egg of the frog. — Gravitation can only 

 indirectly affect life phenomena; namely, when we have iu a 

 cell two different non-miscible liquids (or a liquid and a solid) 



1 Poulton, E. B., Colours of Animals ("International Sciontillc Sories"). 

 London, 1890, p. 121. 



