onological order！。。。The chronological succession of organised forms； the exact determination of those types which appeared first； the simultaneous origin of certain species and their gradual decay； would perhaps teach us as much about the mysteries of organisation as we can possibly learn through experiments with living organisms。〃 （Zittel op。 cit。 page 140。） This； however； was rather the expression of a hope for the distant future than an account of what was attainable； and in practice the science remained almost purely descriptive； until Darwin gave it a new standpoint； new problems and an altogether fresh interest and charm。 The revolution thus accomplished is comparable only to that produced by the Copernican astronomy。
From the first it was obvious that one of the most searching tests of the evolutionary theory would be given by the advance of palaeontological discovery。 However imperfect the geological record might be； its ascertained facts would necessarily be consistent； under any reasonable interpretation； with the demands of a true theory； otherwise the theory would eventually be overwhelmed by the mass of irreconcilable data。 A very great stimulus was thus given to geological investigation and to the exploration of new lands。 In the last forty years； the examination of North and South America； of Africa and Asia has brought to light many chapters in the history of life； which are astonishingly full and complete。 The flood of new material continues to accumulate at such a rate that it is impossible to keep abreast of it； and the very wealth of the collections is a source of difficulty and embarrassment。 In modern palaeontology phylogenetic questions and problems occupy a foremost place and； as a result of the labours of many eminent investigators in many lands； it may be said that this science has proved to be one of the most solid supports of Darwin's theory。 True； there are very many unsolved problems； and the discouraged worker is often tempted to believe that the fossils raise more questions than they answer。 Yet； on the other hand； the whole trend of the evidence is so strongly in favour of the evolutionary doctrine； that no other interpretation seems at all rational。
To present any adequate account of the palaeontological record from the evolutionary standpoint； would require a large volume and a singularly unequal； broken and disjointed history it would be。 Here the record is scanty； interrupted； even unintelligible； while there it is crowded with embarrassing wealth of material； but too often these full chapters are separated by such stretches of unrecorded time； that it is difficult to connect them。 It will be more profitable to present a few illustrative examples than to attempt an outline of the whole history。
At the outset； the reader should be cautioned not to expect too much； for the task of determining phylogenies fairly bristles with difficulties and encounters many unanswered questions。 Even when the evidence seems to be as copious and as complete as could be wished； different observers will put different interpretations upon it； as in the notorious case of the Steinheim shells。 （In the Miocene beds of Steinheim； Wurtemberg； occur countless fresh…water shells； which show numerous lines of modification； but these have been very differently interpreted by different writers。） The ludicrous discrepances which often appear between the phylogenetic 〃trees〃 of various writers have cast an undue discredit upon the science and have led many zoologists to ignore palaeontology altogether as unworthy of serious attention。 One principal cause of these discrepant and often contradictory results is our ignorance concerning the exact modes of developmental change。 What one writer postulates as almost axiomatic； another will reject as impossible and absurd。 Few will be found to agree as to how far a given resemblance is offset by a given unlikeness； and so long as the question is one of weighing evidence and balancing probabilities； complete harmony is not to be looked for。 These formidable difficulties confront us even in attempting to work out from abundant material a brief chapter in the phylogenetic history of some small and clearly limited group； and they become disproportionately greater； when we extend our view over vast periods of time and undertake to determine the mutual relationships of classes and types。 If the evidence were complete and available； we should hardly be able to unravel its infinite complexity； or to find a clue through the mazes of the labyrinth。 〃Our ideas of the course of descent must of necessity be diagrammatic。〃 （D。H。 Scott； 〃Studies in Fossil Botany〃； page 524。 London； 1900。）
Some of the most complete and convincing examples of descent with modification are to be found among the mammals； and nowhere more abundantly than in North America； where the series of continental formations； running through the whole Tertiary period； is remarkably full。 Most of these formations contain a marvellous wealth of mammalian remains and in an unusual state of preservation。 The oldest Eocene （Paleocene） has yielded a mammalian fauna which is still of prevailingly Mesozoic character； and contains but few forms which can be regarded as ancestral to those of later times。 The succeeding fauna of the lower Eocene proper （Wasatch stage） is radically different and； while a few forms continue over from the Paleocene； the majority are evidently recent immigrants from some region not yet identified。 From the Wasatch onward； the development of many phyla may be traced in almost unbroken continuity； though from time to time the record is somewhat obscured by migrations from the Old World and South America。 As a rule； however； it is easy to distinguish between the immigrant and the indigenous elements of the fauna。
From their gregarious habits and individual abundance； the history of many hoofed animals is preserved with especial clearness。 So well known as to have become a commonplace； is the phylogeny of the horses； which； contrary to all that would have been expected； ran the greater part of its course in North America。 So far as it has yet been traced； the line begins in the lower Eocene with the genus Eohippus； a little creature not much larger than a cat； which has a short neck； relatively short limbs； and in particular； short feet； with four functional digits and a splint…like rudiment in the fore…foot； three functional digits and a rudiment in the hind…foot。 The forearm bones （ulna and radius） are complete and separate； as are also the bones of the lower leg （fibula and tibia）。 The skull has a short face； with the orbit； or eye…socket； incompletely enclosed with bone； and the brain…case is slender and of small capacity。 The teeth are short… crowned； the incisors without 〃mark；〃 or enamel pit； on the cutting edge； the premolars are all smaller and simpler than the molars。 The pattern of the upper molars is so entirely different from that seen in the modern horses that； without the intermediate connecting steps； no one would have ventured to derive the later from the earlier plan。 This pattern is quadritubercular； with four principal； conical cusps arranged in two transverse pairs； forming a square； and two minute cuspules between each transverse pair； a tooth which is much more pig…like than horse…like。 In the lower molars the cusps have already united to form two crescents； one behind the other； forming a pattern which is extremely common in the early representatives of many different families； both of the Perissodactyla and the Artiodactyla。 In spite of the manifold differences in all parts of the skeleton between Eohippus and the recent horses； the former has stamped upon it an equine character which is unmistakable； though it can hardly be expressed in words。
Each one of the different Eocene and Oligocene horizons has its characteristic genus of horses； showing a slow； steady progress in a definite direction； all parts of the structure participating in the advance。 It is not necessary to follow each of these successive steps of change； but it should be emphasised that the changes are gradual and uninterrupted。 The genus Mesohippus； of the middle Oligocene； may be selected as a kind of half…way stage in the long progression。 Comparing Mesohippus with Eohippus； we observe that the former is much larger； some species attaining the size of a sheep； and has a relatively longer neck； longer limbs and much more elongate feet； which are tridactyl； and the middle toe is so enlarged that it bears most of the weight； while the lateral digits are very much more slender。 The fore…arm bones have begun to co…ossify and the ulna is greatly reduced； while the fibula； though still complete； is hardly more than a thread of bone。 The skull has a longer face and a nearly enclosed orbit； and the brain…case is fuller and more capacious； the internal cast of which shows that the brain was richly convoluted。 The teeth are still very short…crowned； but the upper incisors plainly show the beginning of the 〃mark〃； the premolars have assumed the molar form； and the upper molars； though plainly derived from those of Eohippus； have made a long stride toward the horse pattern； in that the separate cusps have united to form a continuous outer wall and two transverse crests。
In the lower Miocene the interesting genus Desmatippus shows a further advance in the development of the teeth； which are beginning to assume the long…crowned shape； delaying the formation of roots； a thin layer of cement covers the crowns； and the transverse crests of the upper grinding teeth display an incipient degree of their modern complexity。 This tooth…pattern is strictly intermediate between the recent type and the ancient type seen in Mesohippus and its predecessors。 The upper Miocene genera； Protohippus and Hipparion are； to all intents and purposes； modern in character； but their smaller size； tridactyl feet and somewhat shorter…crowned teeth are reminiscences of their ancestry。
From time to time； when a land…connection between North America and Eurasia was established； some of the successive equine genera migrated to the Old World； but they do not seem to have gained a permanent footing there until the e