NATURE 



121 



THURSDAY, JUNE li, 1891. 



MAMMALS LIVING AND EXTINCT. 

 An Introduction to the Study of Mammals Living and 



Extinct. By W. H. Flower and Richard Lydekker. 



(London : Adam and Charles Black, 1891.) 

 ^T^HIS work is, as the authors inform us in the preface, 



J- based largely upon the article Mammalia, together 

 with forty shorter articles, written by Prof. Flower for 

 the ninth edition of the " Encyclopaedia Britannica." 

 Certain other articles by Dr. Dobson and Dr. St. George 

 Mivart have also been made use of. The illustra- 

 tions, most of which are admirable, are chiefly those 

 prepared for the "Encyclopaedia." but many have been 

 added. Mr. Oldfield Thomas, of the British Museum, 

 has assisted the authors in special matters. To Mr. 

 Lydekker fell the task of arranging the various articles 

 made use of in proper sequence, filling up gaps and 

 adding new matter, a large amount of which treats of the 

 extinct forms. 



The book resulting from this process is undoubtedly 

 one which will be found interesting and useful by all 

 students of zoology. There is a great deal in it which is 

 worth reading ; especially so are the four introductory 

 chapters on general anatomical characters, origin and 

 classification of the Mammalia, and on geographical and 

 geological distribution. Moreover, with regard to im- 

 portant animals such as the horse, sheep, goat, pig, a 

 great deal of accurate information of varied character is 

 given. The whales are, as we should expect, treated 

 with special mastery, and throughout the book we come 

 upon pages which are models of lucid statement and 

 judicious selection of matter. 



It should, however, be clearly understood that the book 

 is not and does not profess to be a complete work of 

 reference on the Mammalia. The references to extinct 

 groups are exceedingly scanty, and whilst they serve to 

 stimulate the reader's desire for further information, do 

 not, as a rule, furnish him with the titles of original works 

 in which such information is to be found. The bulk of 

 the work consists of chapters treating of the orders of 

 Mammalia in systematic sequence, and there is no doubt 

 that, both for the ger>eral reader and for the more technical 

 zoologist, they form a mine of valuable information well 

 up to date, and as a rule well set forth by the aid of 

 illustrations. As an example of the latter, I may refer to 

 the excellent woodcuts of the skull of Tritylodon from 

 the Trias of South Africa, and of various lower jaws 

 illustrating the section on Mesozoic Mammalia ; but ad- 

 ditional figures of this most important and little known 

 series of forms would have been welcome, and one reads 

 with unfeigned disappointment the declaration that "it 

 would be beyond the scope of the present work to 

 describe in detail, or even to mention the names of, all 

 the members of this group." 



There are one or two points of general interest in the 

 earlier chapters to which I may briefly call attention. 



The view originally formulated by Huxley, that in look- 

 ing among Vertebrates for the progenitors of the Mam- 

 malia we must pass over all known forms of birds and 

 reptiles and go right down to the Amphibia, is maintained 

 NO. I 128, VOL. 44] 



by the authors, whilst reconciling this conclusion with 

 Prof. Cope's important observations on the remarkable 

 resemblances which obtain between the extinct reptiles 

 known as Theromorpha (Theriodontia, Pelycosauria) and 

 the Monotreme Mammals. Recent observations have 

 shown such an intimate connection between the South 

 African Theromorpha and the Labyrinthodont Amphibians 

 that there can, our authors maintain, be no hesitation in 

 regarding the one group as the direct descendant of the 

 other, and " we may probably regard the Mammalia as 

 having originated from the same ancestral stock at the 

 time the Amphibian type was passing into the Reptilian." 

 In reference to classification, the authors observe that 

 it is a simple matter to indicate natural groups, such as 

 orders and sub-orders, among existing Mammals, but when 

 we pass to the extinct world all is changed. New forms 

 are discovered which cannot be placed within any of 

 the existing divisions. " Our present divisions and 

 terminology are," say Prof. Flower and Mr. Lydekker, 

 " no longer sufficient for the purpose [of a classification 

 which shall embrace extinct forms] ; and some other 

 method will have to be invented to show the complex 

 relationships existing between different animal forms 

 when viewed as a whole." Apparently the authors 

 mean, by the last five words of this sentence, " when 

 all are viewed together." The necessity for drawing 

 up lists and catalogues in a linear series is deplored, 

 but unhappily no attempt is made by the authors 

 to grapple with the difficulty. A classification of the 

 Mammalia in a linear series is given as far as fami- 

 lies ; and the names of groups containing only extinct 

 forms are printed in special black type. I should have 

 been very glad to see some attempt to set forth in the 

 form of genealogical trees the senior author's views on 

 the genetic relationships of this confessedly artificial 

 linear series. I cannot admit that the division of the 

 Mammalia into three groups— Prototheria, Metatheria, 

 and Eutheria, or, as De Blainville called them, Ornitho- 

 delphia, Didelphia,and Monodelphia— expresses a natural 

 fact, if these three groups are regarded as equipollent, 

 and as succeeding one another as three " grades " of 

 evolution. It is not difficult to come nearer to an ex- 

 pression of actual genealogical relations than this. It 

 appears preferable to divide the Mammalia primarily into 

 two grades : (A.) the Monotrema, and (B.) the Ditrema ; 

 only so do we give expression to the wide gap by which 

 the archaic characteristics of the Monotremes separate 

 them from all other Mammals. Then we can divide the 

 Ditrema — not into two successive grades of structure — but 

 into two diverging branches^ viz. Branch a. Marsupialia, 

 and Branch h. Placentalia. Of the Placentalia our 

 authors say that their affinities with one another are so 

 complex that it is impossible to arrange them serially with 

 any regard to natural affinities. They might, however, 

 it seems to me, embody their own conclusions in classi- 

 ficatory form, and divide the Placentalia into four diverg- 

 ing sub-branches, the chief being {a) the Typidentata, the 

 three others being {b) the Edentata, (t) the Cetacea, and 

 id) the Sirenia. The group which I call Typidentata our 

 authors actually define, though they do not name it and 

 use it as would surely be convenient. They say, " The 

 remaining Eutherian Mammals \i.e. Placentals after ex- 

 clusion of Edentata, Cetacea, and Sirenia] are clearly united 



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