Oct. 8,1874] 



NATURE: 



459 



rocks are, in Great Britain and Ireland and elsewhere, 

 deficient in lime. In our own experience we have seen 

 most valuable results produced by the application of lime 

 to these soils ; and we learn from M. Burat that by the 

 same means several districts in the West of France, which 

 formerly were unable to maintain their people without 

 extraneous supplies of food, have (i.e. by the use of lime) 

 become the largest exporters of grain. All the author's 

 illustrations are taken from France, but they have their 

 counterparts in these islands. 



On the whole, we are justified in saying that the little 

 work will well repay perusal. 



OUR BOOK SHELF 

 Flora of Dorsetshire. By J. C. Mansel-Pleydell. (Lon- 

 don : Whittaker and Co. Blandford : W. Shipp.) 

 Flora Cravonicnsis : or, a Flora of the }'ici)iity of Set lie 

 in Craven, Yorkshire. By John Windsor. (Manches- 

 ter : Cave and Sever, 1S73. Printed for private 

 circulation.) 

 Although the boundary-lines of our counties are, as a 

 rule, purely arbitrary, it is probably wise for the compilers 

 of local floras to maintain them rather than to erect new 

 ones of their own. The area of their observations is, at 

 all events, thus rendered perfectly clear and certain. 

 Dorset has long been famous for its pala'ontological 

 wealth, both vegetable and animal ; and we have here a 

 record of its living flora, which, as might be expected 

 from its length of sea-board and its variety of geological 

 formations — lias, oolite, forest marble, Oxford clay, coral 

 rag, Kimmeridge clay, Portland sand, Purbeck, chalk, and 

 Eocene — is a rich one. The value of local floras 

 depends greatly on the dependence that can be placed 

 on the determination of the species by the editor and his 

 collaborateurs ; and on this point it seems to us that the 

 present work can be safely trusted, great pains having 

 been taken to establish the authenticity both of the locali- 

 ties and of the nomenclature. The county is divided 

 into seven districts determined by the drainage, and 

 therefore generally separated by high land ; and a very 

 good map of the county accompanies the volume. Among 

 the greatest botanical rarities of the county (some of them 

 almost unique) are — Polycarpon tetrafliylluiii, Lotus his- 

 fiili/s, .S'ii/iethis hicolor, Leucojimi vernuin (doubtfully 

 native), G;;v.!' elaiidestiiia, Seirpiis parvulns, and Cynodon 

 dactylon. The flora is confined to flowering plants and 

 vascular cryptogams. 



Mr. Windsor's " Flora of Craven " (the veteran author 

 did not live to see its publication, or rather printing) is 

 compiled on a different plan, the area being a somewhat 

 arbitrary one : "about Settle and its neighbourhood to a 

 moderate distance, generally within twelve miles, but in a 

 few instances extending somewhat further." The district 

 is a remarkably interesting one, whether from a geological 

 or a botanical point of view ; and the flora has been com- 

 piled with as great care as in the other case under notice, 

 with the assistance of several good local botanists, and 

 includes not only the flowering plants and vascular cryp- 

 togams, but also the Characciv, Mosses, HepaticK, and 

 Lichens. A district that includes among its native 

 plants such rarities as Poleiiionium candeuin, Epipaclis 

 ovalis, and Cypripedium caleeoliis, is of no ordinary 

 interest. 



Both these volumes are useful contributions to our 

 library of local botany. We would especially commend 

 to compilers of similar works the plan adopted by Mr. 

 Mansel-Pleydell, of giving the geographical range of each 

 species in the neighbouring counties of England and on 

 the opposite coast of France. 



A, W. B. 



LETTERS TO THE EDITOR 



[ The Editor does not hold liiniself responsible for opinions exfyessed 

 by his correspondents. No notice is taken of anonymous 

 communications .] 



Migration of Birds 



The subject to which Prof. Nenton li.is called attention is one 

 of great interest to all naturalist?, and requires to be studied 

 systematically ; for I can hardly think that the solution is so 

 "simple in the extreme" as Mr. Newton thinks it may be. 



It appears to me probable that here, as in so many other cases, 

 " survival of the fittest " will be found to have had a powerful 

 inlluence. Let us suppose that in any species of migratory bird, 

 breeding can as a rule be only safely accomplished in a given 

 area ; and further, that during a great part of the rest of the year 

 sufficient food cannot be obtained in tliat area. It will follow 

 that those birds which do not leave the breeding area at the 

 proper season will suffer, and ultimately become extinct ; which 

 will also be the fate of those whxh do not kave the .feeding 

 area at the proper time. Now, if we suppose that the two areas 

 were (for some remote ancestor of the existing species) coincident, 

 but by geological and climatic changes gradually diverged from 

 each other, we can easily understand how the habit of incipient 

 and partial migration at the proper seasons would at last become 

 hereditary, and so fixed as to be what we term an instinct. It 

 will probably be found, that every gradation still exists in various 

 parts of the world, from a complete coincidence to a complete 

 separation of the breeding and lire subsistence areas ; and when 

 the natural history of a sufficient number of species in all parts 

 of the world is thoroughly worked out, we may find every Jink 

 between species which never leave a restricted area in which 

 they breed and live the whole year round, to those other cases in 

 which the two areas are absolutely separated. The actual causes 

 that determine the exact time, year by year, at which certain 

 species migrate, will of course be difficult to ascertain. I would 

 suggest, however, that they will be found to depend on thos2 

 climatal changes which most affect the particular species. The 

 change of colour, or the fall, of certain leaves ; the change to 

 the pupa state of certain insects ; prevalent winds or rains ; or 

 even the decreased temperature of the earth and water, may all 

 have their infiuence. Ample materials must exist, in the case of 

 European birds, for an irs'.ructive work on this subject, The 

 two areas should be carefully determined for a number of migia- 

 toiy birds ; the times of their movements should be compared 

 with a variety of natural phenomena likely to infiuence theai ; 

 the past changes of surface, of climate, and of vegetation should 

 be taken account of ; and there seems no reason to doubt that 

 such a mode of research would throw much light on, if it did no: 

 completely solve, the problem. 



This is an appropriate opportunity for" making a suggestion 

 which has long been in my mind. It is, that it would be a 

 valuable and interesting addition to Nature, if we were supplied 

 with a weekly (or monthly) " Calendar of Periodical Phenomena 

 in Natural History," such as the average dates of appearance 

 and departure of migr.atory birds, of the opening and fall of the 

 leaf of our forest trees and common cultivated trees and shrubs ; 

 of the flowering of our common field and garden plants ; and 

 also the mean higlnst and to-.ecst temperature of each day, the 

 direction of the wind and amount of rainfall for each iveel.:, 

 according to the Greenwich averages. None of this information 

 is given in the usual almanacks or periodicals, and it is by no 

 means easy to find it when wanted. Yet it is surely of much 

 value to everjone who lives in the country, and would be the 

 means of exciting an intelligent interest in such observations and 

 in(iuiries as those to which I'lof. Newton has called our attention 

 in his interesting article. Alfreii R. W.-iLLACE 



Regular Motion' in Clockv.?ork 

 In order to ensure perfectly regular motion in the clockwork 

 which drives the revolving diopinc apparatus made by Messes. 

 Chance, Eros, and Co., I have recently introduced a centrifugal 

 governor, which might perhaps also be useful for the clocks of 

 equatorials. Though it involves nothing new in principle, the 

 form differs from anything I have seen, in that the governor balls 

 have to lift a heavy weight, and that the leather rubbers or 

 brushes are not carried by the revolving balls, but are fixed to 

 the frame of the clock and rub against the disc which forms the 

 extra weight lifted by the balls, The sketch shows the governor 



