Effect of colour 261 



two pigments in their chroinatophores, a brown pigment and a whitish 

 or yellow pigment ; the former is much more plentiful than the latter. 

 When the animal appears transparent all the pigment is contained in 

 the centre of the cells, while the ramifications are free from pigment. 

 When the animal appears brown both pigments are spread out into 

 the ramifications. In the condition of maximal spreading the animals 

 appear black. 



This is a comparatively simple case. Much more complicated 

 conditions were found by Keeble and Gamble in other crustaceans, 

 e.g. in Hippolyte cranchii, but the influence of the surroundings upon 

 the colouration of this form was also satisfactorily analysed by these 

 authors. 



While many animals show transitory changes in colour 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 experiments are so well known that a 

 short reference to them will suffice. Poulton 1 found that in gilt 

 or white surroundings the pupae became light coloured 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 lustre. 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 surroundings 

 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 surroundings 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 aw r are, no physico- 

 chemical explanation, except possibly Wiener's suggestion of colour- 

 photography by mechanical colour adaptation, has ever been 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 in a cell two different non-miscible liquids (or a liquid 

 and a solid) of different specific gravity, so that a change in the 

 position of the cell or the organ may give results which can be traced 

 to a change in the position of the two substances. This is very nicely 



1 Poulton, E. B., Colours of Animals (The International Scientific Series), London, 

 1890, p. 121. 



