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rly Tertiary times。 For getting to the Antilles and into Mexico it would have to wait until the Miocene； but long before that time it could arrive in Africa； there surviving as a Congolese and a Cape species。 This story is unsupported by a single fossil。 Peripatus may have been 〃sub…universal〃 all over greater Gondwana land in Carboniferous times； and then its absence from Madagascar would be difficult to explain； but the migrations suggested above amount to little considering that the distance from Tasmania to South America could be covered in far less time than that represented by the whole of the Eocene epoch alone。
There is yet another field； essentially the domain of geographical distribution； the cultivation of which promises fair to throw much light upon Nature's way of making species。 This is the study of the organisms with regard to their environment。 Instead of revealing pedigrees or of showing how and when the creatures got to a certain locality； it investigates how they behaved to meet the ever changing conditions of their habitats。 There is a facies； characteristic of； and often peculiar to； the fauna of tropical moist forests； another of deserts； of high mountains； of underground life and so forth； these same facies are stamped upon whole associations of animals and plants； although these may beand in widely separated countries generally aredrawn from totally different families of their respective 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 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 quicklyindeed this very quickness seems to have initiated our mistaking the variations called forth for something performedand 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 with his brother Erasmus in London； and； after his brother's death； with his daughter； Mrs Litchfield。 On these occasions； it was his habit to arrange meetings with Huxley； to talk over zoological questions； with Hooker； to discuss botanical problems； and with Lyell to hold conversations on geology。 After the death of Lyell； Darwin； knowing my close intimacy with his friend during his later years； used to ask me to meet him when he came to town； and 〃talk geology。〃 The 〃talks〃 took place sometimes at Jermyn Street Museum； at other times in the Royal College of Science； South Kensington； but more frequently； after having lunch with him； at his brother's or his daughter's house。 On several occasions； however； I had the pleasure of visiting him at Down。 In the postscript of a letter （of April 15； 1880） arranging one of these visits； he writes： 〃Since poor； dear Lyell's death； I rarely have the pleasure of geological talk with anyone。〃）
In one of the very interesting conversations which I had with Charles Darwin during the last seven years of his life； he asked me in a very pointed manner if I were able to recall the circumstances； accidental or otherwise； which had led me to devote myself to geological studies。 He informed me that he was making similar inquiries of other friends； and I gathered from what he said that he contemplated at that time a study of the causes producing SCIENTIFIC BIAS in individual minds。 I have no means of knowing how far this project ever assumed anything like concrete form； but certain it is that Darwin himself often indulged in the processes of mental introspection and analysis； and he has thus fortunately left usin his fragments of autobiography and in his correspondencethe materials from which may be reconstructed a fairly complete history of his own mental development。
There are two perfectly distinct inquiries which we have to undertake in connection with the development of Darwin's ideas on the subject of evolution：
FIRST。 How； when； and under what conditions was Darwin led to a conviction that species were not immutable； but were derived from pre…existing forms？
SECONDLY。 By what lines of reasoning and research was he brought to regard 〃natural selection〃 as a vera causa in the process of evolution？
It is the first of these inquiries which specially interests the geologist； though geology undoubtedly played a partand by no means an insignificant partin respect to the second inquiry。
When； indeed； the history comes to be written of that great revolution of thought in the nineteenth century； by which the doctrine of evolution； from being the dream of poets and visionaries； gradually grew to be the accepted creed of naturalists； the paramount influence exerted by the infant science of geologyand especially that resulting from the publication of Lyell's epoch…making work； the 〃Principles of Ge