f existing lizards, the Australian species
represented in the photograph attaining a length of four feet. It has a
brown colour with yellow spots, and in spite of its size it is not
conspicuous against certain backgrounds, such as the bark of a tree.]

Sec. 2

Gradual Change of Colour

The common shore-crab shows many different colours and mottlings,
especially when it is young. It may be green or grey, red or brown, and
so forth, and it is often in admirable adjustment to the colour of the
rock-pool where it is living. Experiments, which require extension, have
shown that when the crab has moulted, which it has to do very often when
it is young, the colour of the new shell tends to harmonise with the
general colour of the rocks and seaweed. How this is brought about, we
do not know. The colour does not seem to change till the next moult, and
not then unless there is some reason for it. A full-grown shore-crab is
well able to look after itself, and it is of interest to notice,
therefore, that the variety of coloration is mainly among the small
individuals, who have, of course, a much less secure position. It is
possible, moreover, that the resemblance to the surroundings admits of
more successful hunting, enabling the small crab to take its victim
unawares.

Professor Poulton's experiments with the caterpillars of the small
tortoise-shell butterfly showed that in black surroundings the pupae tend
to be darker, in white surroundings lighter, in gilded boxes golden; and
the same is true in other cases. It appears that the surrounding colour
affects the caterpillars through the skin during a sensitive period--the
twenty hours immediately preceding the last twelve hours of the larval
state. The result will tend to make the quiescent pupae less conspicuous
during the critical time of metamorphosis. The physiology of this
sympathetic colouring remains obscure.

Seasonal Change of Colouring

The ptarmigan moults three times in the year. Its summer plumage is
rather grouselike above, with a good deal of rufous brown; the back
becomes much more grey in autumn; almost all the feathers of the winter
plumage are white. That is to say, they develop without any pigment and
with numerous gas-bubbles in their cells. Now there can be no doubt that
this white winter plumage makes the ptarmigan very inconspicuous amidst
the snow. Sometimes one comes within a few feet of the crouching bird
without seeing it, and this garment of