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. 



