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ct series。
Turning now to the Artiodactyla； we find still another group of mammals； that of the camels and llamas； which has long vanished from North America； yet took its rise and ran the greater part of its course in that continent。 From the lower Eocene onward the history of this series is substantially complete； though much remains to be learned concerning the earlier members of the family。 The story is very like that of the horses； to which in many respects it runs curiously parallel。 Beginning with very small； five…toed animals； we observe in the successive genera a gradual transformation in all parts of the skeleton； an elongation of the neck； limbs and feet； a reduction of the digits from five to two； and eventually the coalescence of the remaining two digits into a 〃cannon…bone。〃 The grinding teeth； by equally gradual steps； take on the ruminant pattern。 In the upper Miocene the line divides into the two branches of the camels and llamas； the former migrating to Eurasia and the latter to South America； though representatives of both lines persisted in North America until a very late period。 Interesting side…branches of this line have also been found； one of which ended in the upper Miocene in animals which had almost the proportions of the giraffes and must have resembled them in appearance。
The American Tertiary has yielded several other groups of ruminant…like animals； some of which form beautifully complete evolutionary series； but space forbids more than this passing mention of them。
It was in Europe that the Artiodactyla had their principal development； and the upper Eocene； Oligocene and Miocene are crowded with such an overwhelming number and variety of forms that it is hardly possible to marshal them in orderly array and determine their mutual relationships。 Yet in this chaotic exuberance of life； certain important facts stand out clearly； among these none is of greater interest and importance than the genealogy of the true Ruminants； or Pecora； which may be traced from the upper Eocene onward。 The steps of modification and change are very similar to those through which the camel phylum passed in North America； but it is instructive to note that； despite their many resemblances； the two series can be connected only in their far distant beginnings。 The pecoran stock became vastly more expanded and diversified than did the camel line and was evidently more plastic and adaptable； spreading eventually over all the continents except Australia； and forming to…day one of the dominant types of mammals； while the camels are on the decline and not far from extinction。 The Pecora successively ramified into the deer； antelopes； sheep； goats and oxen； and did not reach North America till the Miocene； when they were already far advanced in specialisation。 To this invasion of the Pecora； or true ruminants； it seems probable that the decline and eventual disappearance of the camels is to be ascribed。
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 de