rrupted by shallow seas； just as one would expect if； and when； a Brazil…Ethiopian mass of land were breaking up。 Lastly from Central America to the Mediterranean stretches one of the Tertiary tectonic lines of the geologists。 Here also the great question is how long this continent lasted。 Apparently the South Atlantic began to encroach from the south so that by the later Cretaceous epoch the land was reduced to a comparatively narrow Brazil…West Africa； remnants of which persisted certainly into the early Tertiary； until the South Atlantic joined across the equator with the Atlantic portion of the 〃Thetys；〃 leaving what remained of South America isolated from the rest of the world。
ANTARCTIC CONNECTIONS。 Patagonia and Argentina seem to have joined Antartica during the Cretaceous epoch； and this South Georgian bridge had broken down again by mid…Tertiary times when South America became consolidated。 The Antarctic continent； presuming that it existed； seems also to have been joined； by way of Tasmania； with Australia； also during the Cretaceous epoch； and it is assumed that the great Australia…Antarctic… Patagonian land was severed first to the south of Tasmania and then at the South Georgian bridge。 No connection； and this is important； is indicated between Antarctica and either Africa or Madagascar。
So far we have followed what may be called the vicissitudes of the great Permo…Carboniferous Gondwana land in its fullest imaginary extent； an enormous equatorial and south temperate belt from South America to Africa； South India and Australia； which seems to have provided the foundation of the present Southern continents； two of which temporarily joined Antarctica； of which however we know nothing except that it exists now。
Let us next consider the Arctic and periarctic lands。 Unfortunately very little is known about the region within the arctic circle。 If it was all land； or more likely great changing archipelagoes； faunistic exchange between North America； Europe and Siberia would present no difficulties； but there is one connection which engages much attention； namely a land where now lies the North temperate and Northern part of the Atlantic ocean。 How far south did it ever extend and what is the latest date of a direct practicable communication； say from North Western Europe to Greenland？ Connections； perhaps often interrupted； e。g。 between Greenland and Labrador； at another time between Greenland and Scandinavia； seem to have existed at least since the Permo…Carboniferous epoch。 If they existed also in late Cretaceous and in Tertiary times； they would of course easily explain exchanges which we know to have repeatedly taken place between America and Europe； but they are not proved thereby； since most of these exchanges can almost as easily have occurred across the polar regions； and others still more easily by repeated junction of Siberia with Alaska。
Let us now describe a hypothetical case based on the supposition of connecting bridges。 Not to work in a circle； we select an important group which has not served as a basis for the reconstruction of bridges； and it must be a group which we feel justified in assuming to be old enough to have availed itself of ancient land…connections。
The occurrence of one species of Peripatus in the whole of Australia； Tasmania and New Zealand （the latter being joined to Australia by way of New Britain in Cretaceous times but not later） puts the genus back into this epoch； no unsatisfactory assumption to the morphologist。 The apparent absence of Peripatus in Madagascar indicates that it did not come from the east into Africa； that it was neither Afro…Indian； nor Afro…Australian； nor can it have started in South America。 We therefore assume as its creative centre Australia or Malaya in the Cretaceous epoch； whence its occurrence in Sumatra； Malay Peninsula； New Britain； New Zealand and Australia is easily explained。 Then extension across Antarctica to Patagonia and Chile； whence it could spread into the rest of South America as this became consolidated in early 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 wr