NA TURE 



135 



THURSDAY, NOVEMBER 30, 191 1. 



THE SCIENTIFIC SrVDY OF MAMMALS. 

 I'he Age of Mammals in Europe, Asia, and North 

 America. By Prof. Henry Fairfield Osborn. Pp. 

 xvii + 635, with illustrations. (New York: The 

 -Macmillan Company; London : Macmillan and Co., 

 Ltd., 19 10.) Price 185. 6d. net. 



DURING recent years no branch of study has made 

 more important contributions to biology than 

 that of extinct mammals. It has not only led to a 

 more satisfactory understanding of the mammals 

 themselves and their relationships. It has also thrown 

 unexpected light on the general processes of organic 

 evolution and the problems of geographical distribu- 

 tion. The broad features in the secular development of 

 several groups of mammals are now better known 

 than the growth stages in the individual life-history 

 of many common existing species ; and the underlying 

 principles are often discoverable from a consideration 

 of the numerous recurring phenomena which are 

 sufficiently well known for comparison. 



The literature of the subject, however, has become 

 so voluminous and scattered that the time has 

 arrived for an exhaustive critical summary. The 

 aimless casual descriptions of so-called new species and 

 varieties can only be superseded by real contributions 

 to science when the present position of the various 

 problems is clearly understood. Zoologists, especially 

 palaeontologists, are therefore much indebted to Prof. 

 H. F. Osborn for the great labour he has bestowed 

 on his new work, "The Age of Mammals," which 

 furnishes the necessary summary by a master-hand 

 and forms a sure basis for future research. 



Prof. Osborn 's volume is intended for general scien- 

 tific readers as well as for specialists. It is thus 

 prefaced by an interesting introductory section on the 

 principles of palaeontology as illustrated by extinct 

 mammals, and on certain related geological questions. 

 The treatment is more or less historical, with full 

 references to the original authors, and there are 

 several explanatory diagrams and maps. A table of 

 strata (Fig. 13) marking the periods of successive 

 earth-disturbances which gave rise to various moun- 

 tain-systems, is especially striking. During the "Age 

 of Mammals," or Tertiary period, the Pyrenees, Swiss 

 Alps, and Himalayas have been formed in the Old 

 World, while the Rocky Mountains have originated in 

 North .\merica. 



It is well known that mammals first arose during 

 the Secondary period, or "Age of Reptiles," but re- 

 mained insignificant and restricted in their range until 

 practically all the dominant reptiles had passed away. 

 Prof. Osborn alludes more than once to this appar- 

 ently -^uddcii world-w idi- extinction of the dinosaurs, 

 ichth\()s,uirs, plesiosaurs, and pterodactyls which 

 floul-i^h( (I until the end of Cretaceous times; but it is 

 onl\ lo (inphasise the incNplicable nature c)f the 

 phciKiinc lion. As he nni.nk^, 



"Reptiles are so sensitive to temperature tli.it it is 

 natural to attribute this extinction to a g( n< ral rc- 

 frii,f'|-;iti()n, but the flora shows no evidence ot this 



\'). 2196, VOL. 88] 



either in Europe or America ; nor is there evidence 

 of any great geographic cataclysm on the surface of 

 the earth, for the plant-life transition from one age 

 to the other in the Rocky Mountain region is alto- 

 gether gradual and gentle." 



Whatever may have been the cause, so soon as the 

 great reptiles had disappeared, small primitive mam- 

 mals of the kind which arose in the Secondary period 

 suddenly began to multiply, and spread both in 

 Europe and North America, perhaps also in South 

 America. Their remains are found in the Basal 

 Eocene deposits. None of these animals, however, 

 appear to be directly ancestral to more modern groups, 

 the direct forerunners of which arrived both in Europe 

 and North America from some undetermined region 

 in the period of the Lower Eocene. This second 

 mysterious migration furnished the source of the 

 lemurs, insectivores, true carnivores, rodents, and 

 perissodactyl and artiodactyl ungulates. A few of the 

 primitive mammals still survived with them through 

 the Eocene until the beginning of Oligocene times, 

 most of them grown unwieldy in size, such as the 

 herbivorous Coryphodon and Uintatherium, or the 

 carnivorous Mesonyx ; but their brain remained small 

 and simple, and they could not compete with the 

 higher mammals in which advance in brain-power 

 accompanied progressive elaborations in the limbs and 

 dentition. 



During the Oligocene period, sabre-toothed cats, 

 dogs, martens, pigs, and rhinoceroses became 

 recognisable, both in Europe and North America, 

 while the viverrines occurred in Europe, and the hares 

 and primitive camels were characteristic of North 

 America. Africa was then a separate land-area, the 

 scene of the early development of hyracoids and pro- 

 boscideans. In all regions there were certain preco- 

 cious and aberrant types, such as the titanotheres of 

 North America and Arsinoitherium in Egypt, which 

 only flourished for a short time, without leaving 

 descendants. At the beginning of the Miocene period 

 the most noteworthy event was the connection of 

 Africa with Europe, which allowed the proboscideans 

 to spread over the whole of the northern hemisphere, 

 where they flourished and increased in size. There 

 were now true cats and bears, tapirs, and rhinoceroses, 

 both in the Old and New Worlds; while by the end 

 of the Miocene the horses had nearly become one- 

 toed, and apes, antelopes, okapis, and horned deer 

 had appeared, at least in the Old World. 



By the end of the Pliocpne period mammals had be- 

 come much as they are now, though most of the grouos 

 were more widely distributed, and they comprised many 

 large species which soon disappeared after the advent 

 of man. From the early Eocene until the early Plio- 

 cene, South America had been an isolated land-area, 

 on which mammals had developed into several strange 

 groups of ungulates and the true edentates (sloths, 

 armadillos, and ant-eaters). Before the end of the 

 Pliocene, the emergence of the isthmus of Panama 

 permitted some of these types to wander nortli int<- 

 the southern United States, while the camels (llamas 

 and alpacas), cats, dogs, deer, pigs, horses, and mas- 

 todons were able for the first time to spread to the 

 southern continent. At the end of the Pliocene pi-riod 



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