e
deserts must have lizards capable of standing the glare, the great
changes of temperature, of running over or burrowing into the loose
sand. When as in America Iguanids are available, some of these are thus
modified, while in Africa and Asia the Agamids are drawn upon. Both in
the Damara and in the Transcaspian deserts, a Gecko has been turned into
a runner upon sand!

We cannot assume that at various epochs deserts, and at others moist
forests were continuous all over the world. The different facies and
associations were developed at various times and places. Are we to
suppose that, wherever tropical forests came into existence, amongst
the stock of humivagous lizards were always some which presented those
nascent variations which made them keep step with the similarly nascent
forests, the overwhelming rest being eliminated? This principle would
imply that the same stratum of lizards always had variations ready to
fit any changed environment, forests and deserts, rocks and swamps.
The study of Ecology indicates a different procedure, a great, almost
boundless plasticity of the organism, not in the sense of an exuberant
moulding force, but of a readiness to be moulded, and of this the
"variations" are the visible outcome. In most cases identical facies
are produced by heterogeneous convergences and these may seem to be but
superficial, affecting only what some authors are pleased to call the
physiological characters; but environment presumably affects first those
parts by which the organism comes into contact with it most directly,
and if the internal structures remain unchanged, it is not because these
are less easily modified but because they are not directly affected.
When they are affected, they too change deeply enough.

That the plasticity should react so quickly--indeed this very quickness
seems to have initiated our mistaking the variations called forth for
something performed--and to the point, is itself the outcome of the long
training which protoplasm has undergone since its creation.

In Nature's workshop he does not succeed who has ready an arsenal of
tools for every conceivable emergency, but he who can make a tool at the
spur of the moment. The ordeal of the practical test is Charles Darwin's
glorious conception of Natural Selection.

XVIII. DARWIN AND GEOLOGY. By J.W. Judd, C.B., LL.D., F.R.S.

(Mr Francis Darwin has related how his father occasionally came up from
Down to spend a few days