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, 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 (Poulton, E.B., "Colours of
Animals" (The International Scientific Series), London, 1890, page 121.)
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 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