July 29, 1920] 



NATURE 



67, 



The book is illustrated by a series of thirty-one 

 plates, eight of them in colour and devoted to 

 figures of the protected species, while the rest 

 are black-and-white figures showing the differ- 

 ences between the various species of chats, and 

 certain warblers and wagtails, occurring in the 

 country. 



Investigations in the Theory of Hyperion. By 

 Dr. J. Woltjer, jun. Pp. iii-i-71. (Leyden : 

 E. J. Brill, 1918.) 

 The motion of Hyperion, the seventh satellite of 

 Saturn, is of special interest from the commen- 

 surability of its period with that of Titan, the 

 two being in the ratio of 4 to 3. The late Prof. 

 Newcomb wrote an important paper, "On the 

 Motion of Hyperion : a New Case in Celestial 

 Mechanics." The present work carries the in- 

 vestigation some steps further. The differential 

 equations are broken up into partial systems, 

 giving the inequalities proportional to the succes- 

 sive powers of e' the eccentricity of Titan's orbit. 

 Newcomb had regarded this development as im- 

 practicable, but the present work demonstrates 

 the contrary. The development is at present 

 carried only to the first power of e', which suffices 

 to give a close approximation to the observed 

 inequalities. For example, the ratio of masses of 

 Saturn and Titan is deduced as 3986, which is 

 close to the values 4172, 4125 found by Eichel- 

 berger and Santer respectively. Incidentally, we 

 may note that the mass of Titan is about twice 

 that of the moon, and two-thirds that of Mercury. 

 Also the coefficient of the large inequality that 

 depends on Titan's eccentricity is found as 12-96°, 

 not differing much from the value 14-0° found by 

 H. Struve from observation. 



The researches made by Dr. Woltjer form a 

 useful step in the attainment of a complete theory 

 of Hyperion's motion, and it is to be hoped that 

 he will himself continue the work, carrying it far 

 enough to include all sensible terms. 



A. C. D. Crommelin. 



A Field and Laboratory Guide in Physical Nature- 

 study. By Prof. Elliot R. Downing. (The Uni- 

 versity of Chicago Nature-study Series.) 

 Pp. 109. (Chicago, Illinois : The University of 

 Chicago Press ; London : The Cambridge Uni- 

 versity Press, 1920.) Price i dollar net. 

 At first sight it is difficult to decide whether the 

 book was written for children or for adults — in 

 its assumption of previous knowledge it is hope- 

 lessly above the one ; in its treatment it is far 

 beneath the other. The preface explains that it 

 is meant for pupil teachers. Directions are given 

 for the making of model aeroplanes, the spinning 

 of tops, etc. But if a youth has missed these 

 delights in his childhood, it is of little use for him 

 to try to find them later on. In training a student 

 to teach children there is no need to treat him as 

 a child himself. Nevertheless, the book is full of 

 good ideas, and many who would find it almost 

 intolerable to use as a laboratory manual would 

 be well repaid for time spent in reading it 

 through. 



NO. 2648, VOL. 105] 



Letters to the Editor. 



IThe Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake to 

 return, or to correspond with the writers of, rejected manu- 

 scripts intended for this or any other part of Nature. JVo 

 notice is taken of anonymous communications.] 



Genera and Species. 



When Linne introduced the practice of giving a 

 generic and specific name to each living organism 

 he probably did not anticipate that the number of 

 names required would run into millions, and with the 

 multiplication of genera and species now encouraged 

 by naturalists some other system of distinction seems 

 desirable. 



Of the five or six thousand stars visible to the 

 naked eye only a few have been named, and the rest 

 have to be content with identification by their con- 

 stellations and by a letter or number. 



Sornething of the same sort might be done for the 

 organic world. The conspicuous and typical examples 

 might retain their names, while both to these and to 

 the remainder a letter or number might be allotted. 



If the numbers followed the chronological order in 

 which the species were discovered or first properly 

 described, a catalogue formed on these lines would in 

 itself convey valuable information. 



Identification by number or symbol would act as a 

 check to the coining of many barbarous words, and also 

 to the annoying repetition of the same specific names 

 in different genera. All true classification should be 

 genealogical — that is, it should depend on the ancestry 

 of the organism classified. Existing knowledge is in- 

 sufficient to achieve such an ideal result, but any 

 system not founded on pedigree is open to the objec- 

 tion of not being "natural." 



There are often great doubts as to where varieties 

 end and species begin, and where such doubt exists 

 it would, in general, be safer to assume that differ- 

 ences are varietal until it has been found by trial that 

 continued interbreeding tends to produce sterility. 



I am informed by authorities well acquainted with 

 the West Indies that this is what happens when half- 

 breed is crossed with half-breed, but not when half- 

 breed mates with white (or better, with black). So 

 far as this evidence goes, it points to something 

 approaching a specific difference between the white 

 and the negro, and many species have been deter- 

 mined on a worse foundation. 



Books on special branches of natural history, while 

 giving some sort of description of the various genera 

 and species, do not (there are a few honourable excep- 

 tions) indicate, or indicate very imperfectly, the 

 grounds on which the generic or other distinctions 

 rest, and it is not uncommon to find differences 

 between admitted varieties of the same species ex- 

 ceeding those between species reputed to be separate. 



To find the reasons for these apparent anomalies 

 by consulting the original papers involves the expendi- 

 ture of much time and trouble, but the information 

 might be compressed into a small space if properly 

 tabulated. A. Mallock. 



9 Baring Crescent, Exeter, July 17. 



The Cluster Pine. 



Pintis pinaster was probably introduced into Madeira 

 about thirty years before the beginning of the nine- 

 teenth century — a hundred years too late to save from 

 wanton extinction the forests of mountain laurel, 

 Cerasus, Persea, juniper, and many a species exclu- 

 sively Madeiran. 



There is a settled method of cultivation. The steep 

 hillsides, ridge, and valley, after the yearly autumn 



