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Recent discoveries in Egypt have thrown much light upon a problem which long baffled the palaeontologist； namely； the origin of the elephants。 （C。W。 Andrews； 〃On the Evolution of the Proboscidea〃； 〃Phil。 Trans。 Roy。 Soc。〃 London； Vol。 196； 1904； page 99。） Early representatives of this order； Mastodons； had appeared almost simultaneously （in the geological sense of that word） in the upper Miocene of Europe and North America； but in neither continent was any more ancient type known which could plausibly be regarded as ancestral to them。 Evidently； these problematical animals had reached the northern continents by migrating from some other region； but no one could say where that region lay。 The Eocene and Oligocene beds of the Fayoum show us that the region sought for is Africa； and that the elephants form just such a series of gradual modifications as we have found among other hoofed animals。 The later steps of the transformation； by which the mastodons lost their lower tusks； and their relatively small and simple grinding teeth acquired the great size and highly complex structure of the true elephants； may be followed in the uppermost Miocene and Pliocene fossils of India and southern Europe。
Egypt has also of late furnished some very welcome material which contributes to the solution of another unsolved problem which had quite eluded research； the origin of the whales。 The toothed…whales may be traced back in several more or less parallel lines as far as the lower Miocene； but their predecessors in the Oligocene are still so incompletely known that safe conclusions can hardly be drawn from them。 In the middle Eocene of Egypt； however； has been found a small； whale…like animal （Protocetus）； which shows what the ancestral toothed…whale was like； and at the same time seems to connect these thoroughly marine mammals with land… animals。 Though already entirely adapted to an aquatic mode of life； the teeth； skull and backbone of Protocetus display so many differences from those of the later whales and so many approximations to those of primitive； carnivorous land…mammals； as； in a large degree； to bridge over the gap between the two groups。 Thus one of the most puzzling of palaeontological questions is in a fair way to receive a satisfactory answer。 The origin of the whalebone…whales and their relations to the toothed…whales cannot yet be determined； since the necessary fossils have not been discovered。
Among the carnivorous mammals； phylogenetic series are not so clear and distinct as among the hoofed animals； chiefly because the carnivores are individually much less abundant； and well…preserved skeletons are among the prizes of the collector。 Nevertheless； much has already been learned concerning the mutual relations of the carnivorous families； and several phylogenetic series； notably that of the dogs； are quite complete。 It has been made extremely probable that the primitive dogs of the Eocene represent the central stock； from which nearly or quite all the other families branched off； though the origin and descent of the cats have not yet been determined。
It should be clearly understood that the foregoing account of mammalian descent is merely a selection of a few representative cases and might be almost indefinitely extended。 Nothing has been said； for example； of the wonderful museum of ancient mammalian life which is entombed in the rocks of South America； especially of Patagonia； and which opens a world so entirely different from that of the northern continents； yet exemplifying the same laws of 〃descent with modification。〃 Very beautiful phylogenetic series have already been established among these most interesting and marvellously preserved fossils； but lack of space forbids a consideration of them。
The origin of the mammalia； as a class； offers a problem of which palaeontology can as yet present no definitive solution。 Many morphologists regard the early amphibia as the ancestral group from which the mammals were derived； while most palaeontologists believe that the mammals are descended from the reptiles。 The most ancient known mammals； those from the upper Triassic of Europe and North America； are so extremely rare and so very imperfectly known； that they give little help in determining the descent of the class； but； on the other hand； certain reptilian orders of the Permian period； especially well represented in South Africa； display so many and such close approximations to mammalian structure； as strongly to suggest a genetic relationship。 It is difficult to believe that all those likenesses should have been independently acquired and are without phylogenetic significance。
Birds are comparatively rare as fossils and we should therefore look in vain among them for any such long and closely knit series as the mammals display in abundance。 Nevertheless； a few extremely fortunate discoveries have made it practically certain that birds are descended from reptiles； of which they represent a highly specialised branch。 The most ancient representative of this class is the extraordinary genus Archaeopteryx from the upper Jurassic of Bavaria； which； though an unmistakable bird； retains so many reptilian structures and characteristics as to make its derivation plain。 Not to linger over anatomical minutiae； it may suffice to mention the absence of a horny beak； which is replaced by numerous true teeth； and the long lizard…like tail； which is made up of numerous distinct vertebrae； each with a pair of quill…like feathers attached to it。 Birds with teeth are also found in the Cretaceous； though in most other respects the birds of that period had attained a substantially modern structure。 Concerning the interrelations of the various orders and families of birds； palaeontology has as yet little to tell us。
The life of the Mesozoic era was characterised by an astonishing number and variety of reptiles； which were adapted to every mode of life； and dominated the air； the sea and the land； and many of which were of colossal proportions。 Owing to the conditions of preservation which obtained during the Mesozoic period； the history of the reptiles is a broken and interrupted one； so that we can make out many short series； rather than any one of considerable length。 While the relations of several reptilian orders can be satisfactorily determined； others still baffle us entirely； making their first known appearance in a fully differentiated state。 We can trace the descent of the sea…dragons； the Ichthyosaurs and Plesiosaurs； from terrestrial ancestors； but the most ancient turtles yet discovered show us no closer approximation to any other order than do the recent turtles； and the oldest known Pterosaurs； the flying dragons of the Jurassic； are already fully differentiated。 There is； however； no ground for discouragement in this； for the progress of discovery has been so rapid of late years； and our knowledge of Mesozoic life has increased with such leaps and bounds； that there is every reason to expect a solution of many of the outstanding problems in the near future。
Passing over the lower vertebrates； for lack of space to give them any adequate consideration； we may briefly take up the record of invertebrate life。 From the overwhelming mass of material it is difficult to make a representative selection and even more difficult to state the facts intelligibly without the use of unduly technical language and without the aid of illustrations。
Several groups of the Mollusca； or shell…fish； yield very full and convincing evidence of their descent from earlier and simpler forms； and of these none is of greater interest than the Ammonites； an extinct order of the cephalopoda。 The nearest living ally of the ammonites is the pearly nautilus； the other existing cephalopods； such as the squids； cuttle…fish； octopus； etc。； are much more distantly related。 Like the nautilus； the ammonites all possess a coiled and chambered shell； but their especial characteristic is the complexity of the 〃sutures。〃 By sutures is meant the edges of the transverse partitions； or septa； where these join the shell… wall； and their complexity in the fully developed genera is extraordinary； forming patterns like the most elaborate oak…leaf embroidery； while in the nautiloids the sutures form simple curves。 In the rocks of the Mesozoic era； wherever conditions of preservation are favourable； these beautiful shells are stored in countless multitudes； of an incredible variety of form； size and ornamentation； as is shown by the fact that nearly 5000 species have already been described。 The ammonites are particularly well adapted for phylogenetic studies； because； by removing the successive whorls of the coiled shell； the individual development may be followed back in inverse order； to the microscopic 〃protoconch；〃 or embryonic shell； which lies concealed in the middle of the coil。 Thus the valuable aid of embryology is obtained in determining relationships。
The descent of the ammonites； taken as a group； is simple and clear； they arose as a branch of the nautiloids in the lower Devonian； the shells known as goniatites having zigzag； angulated sutures。 Late in the succeeding Carboniferous period appear shells with a truly ammonoi