November 1, 1894.] 



KNOWLEDGE 



241 



^^ AN ILLUSTRATED «"<^ 



MAGAZINE OF SCIENCE 



SIMPLY WORDED-EXACTLY DESCRIBED 



LONDON: NOVEMBER 1, 1894. 



CONTENTS. 



The Home of the Rodents. By B. Ltdekkbb, B.A.Cantab., 



F.R.S. ... 



The Bacteria of Phosphorescence. By C. A. Mitchell, 



B.A.Oxon 



The Daddy-longlegs. By E. A. Btttiee, B.A., B.Sc. 



The Canals of Mars. By E. Walter Matji-'dee 



Structure of the Milky Way. By E. Barsaed and A. C. 



Eaxtard ... ... ... •• ••■ 



Notices of Books 



Letters :— .T. E. Gore ; (r. M. Se.vbroke 



How Noxious Gases and Vapours from Alkali and other 



Works are Utilized. By G-eorge ilcGo-svAs, Ph.D. ... 



A Classic Double Star. By Miss A. M. C'leeke 



The Koniscope. By Dr. J. G. McPherson, F.R.S.E. 



The Climate of the Cambrian Period. By Prof. J. 



Lo&an' Loblet, F.G-.S , &c. ... 

 The Face of the Sky for November. By Heebebt 



Sadlee, F.K.A.S 



Chess Column. By C. D. LocoCK, B.A.Oxon 



PAOE 



241 



244 

 245 

 249 



253 

 254 

 255 



256 

 257 

 258 



260 



262 

 263 



THE HOME OF THE RODENTS. 



By E. Lydekker, B.A.Cantab., F.K.S. 



RANGING over all the continents and most of the 

 islands of the globe, and being represented even 

 in marsupial-inhabited Australia, the rodents, or 

 gnawing mammals, form such a thoroughly 

 cosmopolitan group that it seems, at first sight, 

 almost illogical to speak of one country more than another 

 as being their home. Nevertheless, in the course of the 

 present article we hope to be able to convince the reader 

 that South America — whether we take into consideration 

 the tropical forests of Brazil, the grassy plains of Argentina, 

 or the desert wastes of Patagonia — has an undoubted claim 

 to the title in question. We may add, however, that under 

 the name South America we include both Central America 

 and the West Indies, the whole of which form one zoological 

 province. 



■ In a previous article we have liad occasion to allude to 

 the chief peculiarities of the fauna of South America, 

 and it was there stated that among the most characteristic 

 forms of mammals were edentates, opossums, marmosets, 

 and the New World monkeys, together with numerous 

 rodents. It did not, however, come within the province 

 of that article to take the latter group into detailed con- 

 sideration, but as this is a subject of much interest from a 

 distributional point of \-iew, it may well form the theme 

 of a separate essay. Probably the majority of our readers 



know what is meant by a rodent, or gnawing mammal ; 

 but should there be some among them whose ideas are 

 somewhat hazy on this subject, it is to be hoped that their 

 more learned brethren will pardon a few words in regard 

 to the characteristics of the creatures in question. 



As common and well-known examples of the rodent order 

 we may cite squirrels, dormice, marmots, beavers, rats, 

 voles, porcupines, and hares and rabbits, all of which are 

 characterized by possessing a pair of chisel-shaped teeth 

 in the front of each jaw, which are worn by use into a 

 shai-p, cutting, transverse ridge, and grow continuously 

 throughout the life of their owners. It is with these i 

 chisel-like front or incisor teeth that the rodents perform ; 

 that gnawing action (so markedly developed in the beavers 

 and porcupines) from which they derive their name ; and 

 it is owing to the circumstance that the front of each tooth 

 is faced with a plate of hard enamel, while the remainder 

 consists of soft ivory, that these beautiful instruments 

 mamtain their cutting edges. These two pairs of front 

 teeth are absolutely characteristic of all rodents, and in 

 by far the greater majority of the order there are no other 

 teeth in this regiea of the jaw. As if, however, for the 

 purpose of hinting to us how these animals were originally 

 related to mammals.provided with a fuller set of teeth, the 

 hares and rabbits, together with their near allies the picas 

 or tailless hares, have a minute pair of somewhat similar, 

 teeth placed immediately behind the large pair in the upper 

 jaw. Being perfectly useless to their owners, this second 

 pair of upper front teeth evidently comes under the 

 category of rudimental or vestigiary structures. Behind 

 the front teeth of all rodents comes a long gap in each jaw, 

 after which we reach the series of grinding or cheek-teeth, 

 which are never more than six in number, and are 

 frequently reduced to four, or even three. Consequently, 

 no member of the order ever has tusks or canine teeth. 

 Were it not that there are two groups of animals with a 

 dentition of a simUar type, these pecuUarities in the teeth 

 would absolutely distinguish rodents from all other 

 members of the mammalian class. Of the groups in 

 question, the one contains the wombats of Australia, which 

 are broadly distinguished by the presence of a pouch for 

 the young, while the second gi-oup is represented solely by 

 the curious aye-aye of Madagascar, which agrees in its 

 internal anatomy with the lemurs, and is accordingly 

 assigned by naturaUsts to that group. With these 

 exceptions, the dentition is absolutely characteristic of the 

 rodent order ; and as the reader is not likely to confound 

 with them either of the creatures named, he may rely on 

 the nature of the teeth in identifying the members of the 

 order under consideration. 



Ha^^ng thus settled what constitutes a rodent, our next 

 point is the classification of the order ; for until we gain 

 some insight into this, it is quite impossible to imderstand 

 the especial richness of South America in regard to these 

 animals. Eodents, then, are divided by naturaHsts into 

 seventeen families, several of which are brigaded together 

 into a smaU number of larger groups. Of these groups, 

 the first is the squirrel-like rodents, which includes the 

 four famihes of the African flying squirrels {Anomaluridw), 

 the squirrels and • marmots {Scmrvlm), the seweUels 

 (Haplodontiihr), and the beavers {Castorida). Now, of 

 these four famihes, the only one occurring in South 

 America is the second, and its representatives there are 

 merely certain species of squirrels, which, moreover, do not 

 range south of Paraguay ; the whole of the marmots, 

 prairie-dogs, susliks, chipmunks, etc., being totally imknown 

 south of the Isthmus of Darien. 



The second gi-eat group of the order, known as the 

 mouse-like rodents, includes five families — namely, the 



