Effect of colour 261 
two pigments in their chromatophores, 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, 
eg. in Hippolyte cranchiz, 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! 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 
80 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 aware, 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. 
