March 14, 1895] 



NA TURE 



459 



Conspectus Flora Africa, oti Enumeration ties Planles 



d'Afrique Par Th. Durand et HansSchinz. Vol. v. 



Monocotyledoneae et Gymnospermeae. 8vo. Pp.977. 



(Bruxelles, 1895 ) 

 It may, perhaps, be asked why the fifth volume of a 

 ■work should appear before the fourth and all the pre- 

 •ceding ones. Doubtless the authors were influenced 

 thereto by the fact that neither Oliver's " Flora of 

 Tropical Africa," nor Harvey's " Flora Capensis," has 

 reached the groups enumerated in the bulky volume 

 under notice. Certainly this course has the advantage 

 ■of utility, and will be of great service in the elaboration 

 of the continuation of the works named. As an index 

 to the scattered literature of the subject, the present 

 volume is indeed invaluable. It covers all that may be 

 called African, including the Ailantic islands from 

 Madeira to Tristan d'Acunha. and the islands of the 

 Indian Ocean, from St. Paul and Amsterdam to Mauritius, 

 Madagascar, and Socotra. It is true, the geography of the 

 plants is not worked out all through so fully as Mr. C. B. 

 Clarke has done the Cyperaceae. For instance, the 

 characteristic grass of Tristan d'Acunha and St. Paul 

 ind Amsterdam islands, Spartina arundinacea, is only 

 recorded from the former group. In other respects, Mr. 

 Clarke's elaboration of the 800 Cyperaceae is by far the 

 most complete and thorough part of the volume, though 

 it is blemished by the introduction of a very large 

 number of names of new species without descriptions. 



I5ut, leaving all criticism out, this volume will be wel- 

 ' Dined alike by horticulturists and botanists ; by the 

 [inner, more especially, because it contains the petaloid 

 nionocols, so numerous in South Africa. Synonyms and 

 references to figures in the various illustrated serials add 

 to the usefulness of the enumeration. To give an idea 

 of the extent of this compilation, it may be mentioned 

 that the LiliaceK include nearly 1 100 species, belonging 

 to 67 genera. Aloe alone numbers nearly 100 species. 

 The Iridese are about 700 strong; Gladiolus being the 

 largest genus, with 1 43 species. < rchids also exceed 

 1000 species, belonging to 74 genera ; and 160 species 

 of Habenaria are enumerated. Palms are less numerous 

 than might have been expected, considering the com- 

 paratively large number in a small group of islands like 

 the Seychelles. Only 63 species are given, which is 

 about a quarter the number inhabiting British India. 

 This is largely due to the genus Calamus being repre- 

 sented by only one species in Africa, whereas there are 

 72 in India. W. B. H. 



Li-cons de Chiinie. Par H. Gautier et G. Charpy. 



I'Paris: Gauthier-\'illars, 1894.) 

 The general plan of the second edition of this work does 

 not differ essentially from that of its predecessor. The 

 introductory portion on generalities — dealing with states 

 of aggregation, laws of combination, equivalents and 

 atomic weights, physical and chemical transformation, 

 chemical equilibrium, the velocity of reaction, thermo- 

 chemistry, &c. — has been recast, and now occupies one- 

 fifth of the volume. In the descriptive portion, which is 

 concerned with inorganic chemistry only, Moissan's 

 work on fluorine, the diamond, and boron has been 

 introduced. It is characteristic of a French text-book 

 that even now it is deemed necessary to print alongside 

 each important atomic equation the corresponding 

 equation based on equivalents. In the same connection 

 it will be somewhat disconcerting to English students to 

 find that " Le poids atomique est i!gdl au poids 

 equivalent pour les elements suivants : . . . Pour tous 

 les autres elements, la valeur du poids atomique est 

 i double de celle de I'equivalent." In other respects the 

 book is well up to date, and contains much useful 

 information expressed with the clearness and precision 

 for which French text-books are deservedly famed. 



J. W. R. 



J NO. i3?4, VOL. 51] 



LETTERS TO THE EDITOR. 



[T/u Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

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

 manuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications.] 



Variation and Specific Stability. 



I AM afraid that in my anxiety to compress too long a state- 

 men', I did not make the points which I wished to bring 

 forward in the recent discussion at the Royal Society 

 sufficiently clear. I have therefore written out the following 

 summary : — 



( 1 ) All organisms vary. That in doing so they obey Qaete- 

 let's law was suggested by Mr. Darwin himself more than 

 twenty years ago. He observes (Nature, September 25, 

 1873, p 432) : — " It is known from the researches of Quelelet 

 . . . ihat men may be grouped symmetrically about the average 

 with reference to iheir height. . . . We may presume that this 

 is the usual law of variaiion in all the parts of every species 

 under ordinary conditions of life." 



(2) Prof. George Darwin supplemented this in a following 

 number (October 16, p. 505) with a very lucid account of the 

 principle. In this he says : — " We may assume with some 

 confidence that under normal conditions, the variation of any 

 organ in the same species may be symmetrically grouped about 

 a centre of greatest density." 



(3) A well-known illustration is that of a marksman shooting 

 at a target. The dis'ribu'ion of his shots will follow the same 

 law ; they will be grouped round a centre of greatest density, 

 which is easily ascerlainfd, as it is the centre of graviiy of the 

 circumscribed figure. And on successive trials, it all ennriitions 

 remain unaltered, the position of the centre will remain the 

 same, though the positions of the shots will be different. 



(4) No two individual representatives of a species in nature 

 are exactly alike. All differ in some respect. We may picture 

 the aggregate, however, as grouped with respect to any dis- 

 ctiminaiing character like the shots on the target. Our con- 

 ception of the species to which they belong is an abstraction 

 which we endeavour to represent in our museums by a specimen 

 which would be placed as near as possible to the centre of 

 greatest density. Such an abstraction we may call the mean 

 specific form. 



(5) Returning to the case of the target, it is ohviius that if 

 some new condition be introduced, such a^ a wind blowing 

 transversely, every shot will be affected, and the centre of den- 

 sity of the system will be shifted. What is the analogous 

 result when we are dealing with the aggregate of individual 

 organi-ms representing "a species"? 



Natural selection will come into play, to begin with. It 

 may be that some hitherto indifferent variation may be favoured 

 by the new condition. Others will be relatively hanHicapped, 

 and such a favoured variation will get the upper hand. It is 

 obvious that ihe result will be to shift the centre of density : 

 the mean specific form will have undergone a corresponding 

 charge. 



(6) It is probable that so simple a result is not the usual one, 

 and wh.it actually takes place is much more complex. 



Mr. Darwin concludes " that organic beings when subjected 

 during several generations to any change wha ever in their 

 conditions, tend to vary." (" Variation of Animal- and Plants," 

 ii. p. 250.) I infer therefore, and all the lacs which have 

 come under my ob-ervali'>n confiruj it, that a chat'ge in the 

 external conditions, otherwise the environment, will provoke 

 seme variaiion in the organism, which I may call the stimulated 

 variation. 



(7) It appears to me that fiom the Lamarckian point of view, 

 the stimulated variaiion ought to be immediately adaptive. 

 Froiu the Darwinian this is not necess.irily the case. It may- 

 be eiih' r advantageous or, at any rate, indifferent. ("Changed 

 conditions ^eneially induce mere flrciualing variability.'' 

 Darwin, "Origin," 6ih ed. p. 131.) Prof. Ge ir>;e Darwin, 

 in ihe note above citel, traces out the result in the two cases. 

 In the former case, "with continual intercrossing;," the new 

 variai ^n will ge' the upper hand, and the ctniie of density will 

 De shifted ; in the latter it will, by continuous "weeding out," 

 be, after a temporary displacement, eventually restored. 



(8) This leads to the considcraiioii of the stability of the mean 

 specific lorm. Some species seem to yield preliy rapidly, though 

 with an appreciable inertia, to the influence of changed condi- 



