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ng more upon quantity than quality； have obscured the fact that the key to the present distribution lies in the past changes of the earth's surface。 However； with Wallace begins the modern study of the geographical distribution of animals and the sudden interest taken in this subject by an ever widening circle of enthusiasts far beyond the professional brotherhood。
A considerable literature has since grown up； almost bewildering in its range； diversity of aims and style of procedure。 It is a chaos； with many paths leading into the maze； but as yet very few take us to a position commanding a view of the whole intricate terrain with its impenetrable tangle and pitfalls。
One line of research； not initiated but greatly influenced by Wallace's works； became so prominent as to almost constitute a period which may be characterised as that of the search by specialists for either the justification or the amending of his regions。 As class after class of animals was brought up to reveal the secret of the true regions； some authors saw in their different results nothing but the faultiness of previously established regions； others looked upon eventual agreements as their final corroboration； especially when for instance such diverse groups as mammals and scorpions could； with some ingenuity； be made to harmonise。 But the obvious result of all these efforts was the growing knowledge that almost every class seemed to follow principles of its own。 The regions tallied neither in extent nor in numbers； although most of them gravitated more and more towards three centres； namely Australia； South America and the rest of the world。 Still zoologists persisted in the search； and the various modes and capabilities of dispersal of the respective groups were thought sufficient explanation of the divergent results in trying to bring the mapping of the world under one scheme。
Contemporary literature is full of devices for the mechanical dispersal of animals。 Marine currents； warm and cold； were favoured all the more since they showed the probable original homes of the creatures in question。 If these could not stand sea…water； they floated upon logs or icebergs； or they were blown across by storms； fishes were lifted over barriers by waterspouts； and there is on record even an hypothetical land tortoise； full of eggs； which colonised an oceanic island after a perilous sea voyage upon a tree trunk。 Accidents will happen； and beyond doubt many freaks of discontinuous distribution have to be accounted for by some such means。 But whilst sufficient for the scanty settlers of true oceanic islands； they cannot be held seriously to account for the rich fauna of a large continent； over which palaeontology shows us that the immigrants have passed like waves。 It should also be borne in mind that there is a great difference between flotsam and jetsam。 A current is an extension of the same medium and the animals in it may suffer no change during even a long voyage； since they may be brought from one litoral to another where they will still be in the same or but slightly altered environment。 But the jetsam is in the position of a passenger who has been carried off by the wrong train。 Almost every year some American land birds arrive at our western coasts and none of them have gained a permanent footing although such visits must have taken place since prehistoric times。 It was therefore argued that only those groups of animals should be used for locating and defining regions which were absolutely bound to the soil。 This method likewise gave results not reconcilable with each other； even when the distribution of fossils was taken into account； but it pointed to the absolute necessity of searching for former land…connections regardless of their extent and the present depths to which they may have sunk。
That the key to the present distribution lies in the past had been felt long ago； but at last it was appreciated that the various classes of animals and plants have appeared in successive geological epochs and also at many places remote from each other。 The key to the distribution of any group lies in the configuration of land and water of that epoch in which it made its first appearance。 Although this sounds like a platitude； it has frequently been ignored。 If； for argument's sake； Amphibia were evolved somewhere upon the great southern land…mass of Carboniferous times （supposed by some to have stretched from South America across Africa to Australia）； the distribution of this developing class must have proceeded upon lines altogether different from that of the mammals which dated perhaps from lower Triassic times； when the old south continental belt was already broken up。 The broad lines of this distribution could never coincide with that of the other； older class； no matter whether the original mammalian centre was in the Afro…Indian； Australian； or Brazilian portion。 If all the various groups of animals had come into existence at the same time and at the same place； then it would be possible； with sufficient geological data； to construct a map showing the generalised results applicable to the whole animal kingdom。 But the premises are wrong。 Whatever regions we may seek to establish applicable to all classes； we are necessarily mixing up several principles； namely geological； historical； i。e。 evolutionary； with present day statistical facts。 We might as well attempt one compound picture representing a chick's growth into an adult bird and a child's growth into manhood。
In short there are no general regions； not even for each class separately； unless this class be one which is confined to a comparatively short geological period。 Most of the great classes have far too long a history and have evolved many successive main groups。 Let us take the mammals。 Marsupials live now in Australia and in both Americas； because they already existed in Mesozoic times； Ungulata existed at one time or other all over the world except in Australia； because they are post…Cretaceous； Insectivores； although as old as any Placentalia； are cosmopolitan excepting South America and Australia； Stags and Bears； as examples of comparatively recent Arctogaeans； are found everywhere with the exception of Ethiopia and Australia。 Each of these groups teaches a valuable historical lesson； but when these are combined into the establishment of a few mammalian 〃realms；〃 they mean nothing but statistical majorities。 If there is one at all； Australia is such a realm backed against the rest of the world； but as certainly it is not a mammalian creative centre！
Well then； if the idea of generally applicable regions is a mare's nest； as was the search for the Holy Grail； what is the object of the study of geographical distribution？ It is nothing less than the history of the evolution of life in space and time in the widest sense。 The attempt to account for the present distribution of any group of organisms involves the aid of every branch of science。 It bids fair to become a history of the world。 It started in a mild； statistical way； restricting itself to the present fauna and flora and to the present configuration of land and water。 Next came Oceanography concerned