336 Geographical Distribution of Animals 
tive orders. It does not go to the root of the matter to say that 
these facies have been brought about by the extermination of all the 
others which did not happen to fit into their particular environment. 
One might almost say that tropical moist forests must have arboreal 
frogs and that these are made out of whatever suitable material 
happened to be available ; in Australia and South America Hylidae, 
in Africa Ranidae, since there Hylas are absent. The deserts must 
have lizards capable of standing the glare, the great changes of tem- 
perature, 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 con- 
vergences 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 out- 
come 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. 
