294 



NA TURE 



[January 26, 1899 



Symboliie AnlillaiMe : seu Fmuiamenta Florae Indiae 

 Occidenialis. Editit Ignatius Urban. \'ol. i. Kasc. i. 

 Pp. 192. (Berlin : Friedlander, 1898.) 



Since the publication of Grisebach's " Flora of the 

 British West Indian Islands" in 1859-64, large additions 

 have been made to our knowledge of the native plants, 

 not only of our own West Indian islands, but also of those 

 belonging toother countries. The distinguished Curator 

 of the Imperial herbarium at Merlin has contributed 

 much to this knowledge, but in the form of papers 

 scattered through a number of botanical publications. 

 These he now proposes to collect, and to publish in a 

 connected form, together with hitherto unpublished' de- 

 scriptions of new genera and species, &c. The first 

 instalment, of 192 pages, is occupied entirely with a 

 bibliography of West Indian botany, every work being 

 mentioned which furnishes any information on the native 

 products of the West Indies, whether phanerogamic or 

 cryptogamic, with, where possible, an account of its 

 contents. Every one who has worked at local floras will 

 know how work of this kind is facilitated by a good 

 bibliography ; and the thanks of systematic botanists are 

 due to Prof. Urban for the thoroughness with which he 

 has executed this task. 



lOTua Geological Sun'cj. Annual Report, 1897, with 

 accompanying papers. \'oI. viii. Dr. .Samuel Calvin 

 and H. F. Bain. Pp. 427. (Des Moines, 1898). 

 A l,.\RGE part of this volume is taken up with reports on 

 the geology of Dallas, Delaware, Buchanan, Decatur 

 and Plymouth counties, Iowa. With the survey work 

 referred to, the survey and mapping of twenty-si.\ counties 

 in the State have been completed. In addition to this 

 areal work, special studies of coal, clay, artesian waters, 

 gypsum, lead, zinc, &c., have absorbed a considerable 

 portion of the funds and time of the Sur\ey. .As in pre- 

 vious years, close attention was given to the study of 

 problems connected with the drift, and very gratifying 

 progress was made. A paper by Mr. H. F. Bain, 

 Assistant State Geologist, on properties and tests of Iowa 

 building stones, contains much instructive information 

 concerning building stones in general, as well as results 

 of tests of Iowa stones. 



In concluding his summary report, Dr. Calvin remarks : 

 " It is gratifying to note the increased use of the reports 

 of the -Survey, as works of reference, or works for general 

 study, in high schools and other educational institutions. 

 Progressive teachers have been quick to recognise the 

 educational value of trustworthy tests relating to the 

 physical geography and geological phenomena of regions 

 with which the students are personally acquainted." The 

 reports are thus performing a mission of great educational 

 value to the State. 



Numerous plates and half-tone illustrations accompany 

 the papers in the report. 



Elementary Mathematics. By J. L. S. Hatton, M.A., 

 and George Bool, B.A. Pp. viii -f- 356. (London : 

 Whittaker and Co., 1898.) 

 It is not often that arithmetic, Euclid, and algebra, are 

 dealt with in a single text-book, and the only reason for 

 their joint appearance in the present volume, is that 

 students working in classes examined by the Science and 

 Art Department may have at hand a means of qualifying 

 themselves for the May examination in Stage I. Mathe- 

 matics. A knowledge of the fundamental rules of arith- 

 metic is assumed, but numerous examples are given upon 

 them. The Euclid embraces the first Book, with a few 

 additions ; and the algebra extends to problems involving 

 simultaneous equations. If only a small proportion of 

 the examples is worked by the student, the dexterity re- 

 quired to pass the examination for which the book is 

 intended will be obtained. 



NO. 1526, VOL. 59] 



LETTERS TO THE EDI 1 OR. 

 [The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with Ike writers of rejected 

 manuscripts intended for this or any other fart of Nati'RK. 

 iVo notice is taken of anonymous communications. ] 



Prof. Meldola's Explanation. 



I \\l obliged by the courteous expressions of Prof. Meldola's 

 letter, and regret that I should have given him the trouble of 

 writing the latter part of it by not quoting in full a certain 

 passage which I quoted in part : a ileficiency of quotation caused 

 l>y the wish to occupy as little space as possible. .\s evidence 

 of change in my views. Prof. Meldola takes from "The Factors 

 of Organic Evolution " the following passage : — 



"Eventually, among creatures of high organisation, this 

 factor [inheritance of functionally-wrought changes] became an 

 important one ; and I think there is reason to conclude that, in 

 the case of the highest of creatures, civilised men, among whom 

 the kinds of variation which affect survival are loo n;ultitudinous 

 to permit easy selection of any one, and among whom survival of 

 the fittest is greatly interfered with, it has become the chief 

 factor : such aid as survival of the fittest gives, being usually 

 limited to the preservation of those in whom the totality of the 

 faculties has been most favourably moulded by functional 

 changes" (p. 74). 



I now give at length the corresponding passage from the first 

 edition of the " Principles of Biology." 



' ' As fast as the number of organs that co-operate in any given 

 function increases, indirect equilibration through natural selection 

 becomes less and less capable of producing specific adaptations ; 

 and remains fully capable only of maintaining the gener.tl fitness 

 of constitution to conditions. Simultaneously, the pmduction 

 of adaptations by direct equilibration, takes the first place— in- 

 direct equilibration serving to facilitate it. Until at length, 

 among the civilised human races, the equilibration becomes 

 mainly direct : the action of natural selection being restricted to 

 the destruction of those who are constitutionally too feeble to 

 live, even with external aid." 



It will be seen that there is no diflTerence between the two, 

 save in form of expression. My belief remains just what it was 

 in 1S64. 



I suspect that the erroneous impression Prof. Meldola refers 

 to resulted mainly from the ill-judged title "The Inadequacy 

 of Natural Selection." I meant simply to imply that natural 

 selection does not explain all the facts. .\ better title would 

 have been " Natural Selection a part cause only." 



Brighton, January 22. Herbert Spencer. 



Illusory Resolutions of the Lines of a Spectrum. 



Or. Presion, in his useful article on Zeeman's phenomenon 

 in Nature of the 5th inst., has expressed the opinion that some 

 of the resolutions of .spectral Hnes obtained by Michelson's 

 interferometer are illusory (see p. 228). I had occasion some 

 months ago to make use of some of Michelson's results, and came 

 to the same conclusion. In fact, an instrumental resolution of 

 what is in reality a single line may cause it to assume appear- 

 ances, the principal of which are either a central line with i.\m\ 

 appendage lines, or a double line with appendages. 



I. Central line and appendages. — Let us first consider the 

 illumination produced in the first spectrum by a parallel beam 

 of monochromatic light incident on a grating with « + I [■ar.allel 

 grooves, furnishing n equal and equally spaced reflecting strips 

 between the grooves. This produces in the first spectrum what 

 is usually regarded as a line in a position which we may call A, 

 but what is really a distribution of light along the whole 

 spectrum, having (if the slit be of infinitesimal width) positions 

 of cypher inten.sity at short intervals, and having a maximum of 

 illumination between every two consecutive positions of cypher 

 intensity.' The intensities of these maxima are such that one of 



1 If ihc UkIii is incident perpendicularly on .t flalgr.iting with <i reflecting 

 -trips. the principal maximum illumination in the first spectrum occurs in the 

 direction A, which makes, with the normal to the grating,. in angle ». such 

 that sin 6 = A/<r, where A is the wave-lenglh of the monochromatic light, and v 

 the spacing of the reflecting strips of the grating. In the directions*-*- W, 

 where M = *""" ■ the illumination is o if * is an even integer, and a ma\i- 



mum if* is an odd inlcKer : and the intensities are as st.ited in the te.vl. 

 Similar results come ,>ut when the incident light reaches the grating in other . 

 th.in the perpendicular direction, or when the grating is ;.)ncave instead of 



