November 13, 1902] 



NA TURE 



27 



66 Palaearctic, the effect being doubtless mainly due to 

 that exceptionally strong power of flight to which the 

 authors attributed the unusually wide distribution of but- 

 terflies belonging to this family. Other still more interest- 

 ing intruders are the great Danaine butterflies, of which 

 A nosia plexippus, the " Monarch," is the best known and 

 the widest ranging, inasmuch as it extends far into 

 Canada. The peculiar interest of these settlers lies in 

 the fact that certain species of the Holarctic fauna have 

 been profoundly modified into mimicry of them, thus 

 proving beyond the possibility of doubt that the invasion 

 is no new thing — like the spread of the great Danaine 

 plexippus into the Philippines, the Fijis, Australia, Hong 

 Kong, &c. 



The whole of the vast mass of material i n these and 

 the great series of companion memoirs is a remarkable 

 testimony to the insight of P. L. Sclater in drawing the 

 outlines of his regions, of Darwin in laying down the 

 principles of geographical distribution in the " Origin," 

 and of Wallace in his masterly development of the sub- 

 ject in his great works on the geographical distribution of 

 animals. These principles have been tested by an appeal 

 to the facts collected with consummate skill and care 

 from the most critical area in the world, and assuredly 

 they have not been found wanting. 



The work is printed and brought out in the same 

 beautiful and costly style as the rest of the series. It con- 

 tains 112 plates, with more than 2000 admirably executed 

 hand-coloured figures representing 1250 species, and 

 nearly 550 uncoloured figures of the structural parts of 

 butterflies. 



The total number of species of Rhopalocera recognised 

 in Central America as here defined is 1805, as against 

 642 in the New- World and 716 in the Old-World segment 

 of the Holarctic belt. Of these 1805, 360 (almost exactly 

 one-fifth) are described as new. A valuable table of 

 genera shows distinctly and at a glance the relative num- 

 bers of the species in each of the eight districts of Central 

 America, in South America, in North America and in the 

 West Indies. The extraordinary poverty of the fauna of 

 the latter is well brought out by this comparison. 



The classification adopted is mainly that of H. W. 

 Bates in his paper on the insect fauna of the Amazon 

 Valley {Journal of Entomology, ii. pp. 175-185, 1864). 

 The Libytheida:, instead of being included in the 

 Erycinidre, are kept as a separate family, represented 

 in the area under consideration by a single species. 

 One slight criticism may be suggested ; the monograph 

 begins with the most specialised subfamily the 

 Danainas, but within the subfamily itself the more 

 specialised group the Ithomiina follows, instead of 

 precedes, the less specialised Danaina. 



This vast undertaking has required the cooperation 

 of some of the best living collectors of insects. In addi- 

 tion to the visits of the editors, Mr. G. C. Champion, 

 Mr. H. H. Smith and many others have remained in 

 Central America for long periods of time collecting 

 material for the " Biologia." Great collections, such as 

 those of H. W. Bates and Herbert Druce, have been 

 acquired as a whole and added to the mass of material, 

 which was steadily accumulating for forty years. Wherever 

 Central American specimens could be acquired or 

 borrowed, they have been studied for the purpose of this 

 NO. I 724, VOL. 67] 



great work ; the single exception was due to the im- 

 possibility of receiving the loan of Plotz's quoted but 

 unpublished figures. It is unnecessary to say anything 

 further of a difficulty thus gratuitously thrown in the 

 way of a memorable advance in zoological science, a 

 great gift, not restricted to any single nation, but con- 

 ferred upon the learning of the world. 



The thorough treatment of the more obscure Central 

 American groups and genera is such as to render the 

 work absolutely necessary for the study of the related 

 species in other parts of the world. 



It would be inappropriate to discuss the details of this 

 great monograph at any length on the present occasion, 

 but all naturalists should gain a knowledge of the general 

 results, in part briefly discussed in this article, which 

 are lucidly set forth in the introductory chapter. And 

 every naturalist, before he reaches the end of the record 

 of results and conclusions, will feel how deep is the debt 

 that he owes to the research and munificence which 

 have led to so notable a widening of the boundaries of 

 knowledge. E. B. P. 



PRINCIPLES OF DYNAMICS. 



Sur les Principes de la Micanique Ra/ionclle. Par C. de 

 Freycinet, de l'lnstitut. Pp. viii + 167. (Paris : 

 Gauthier-Villars, 1902.) Price fr. 4. 



MDE FREYCINET first became known to the 

 • world as the author of a treatise on dynamics 

 of some bulk and repute, which was published in 1858, 

 and the essay before us shows that at the end of his long 

 and distinguished career of active public life his interest 

 in the subject remains unabated. Referring to his 

 publications during the intervening period, we find two 

 relating to dynamics. In 18S7, he communicated to the 

 Academy of Sciences (Comptes rendus, cv., pp. 903-910) 

 a note containing the rather interesting suggestion that 

 the term " dynamical capacity " should be adopted in the 

 place of the term "density" as derived from dynamical 

 considerations, on the analogy of calorific capacity ; also 

 some proposals about units which were not likely to 

 meet with acceptance. He proposed a standard unit of 

 length derived from the value of gravity at Paris, recom- 

 mending it by the remark that the length of a pendulum 

 can be measured more conveniently than that of a 

 meridian of the earth. In 1896, he published his essays 

 on the philosophy of the sciences, containing some 

 chapters on mechanics. 



The book before us gives the impression of not being 

 up to date, and repetitions from the author's former works 

 which we find in it afford some explanation of this. He 

 does not appear to be well acquainted with the modern 

 literature of the subject. One might expect to find some 

 sign of the influence of Mach, but there is none. The 

 framework of the essay is a constructive sketch of the 

 subject, which cannot be regarded as of much value. It 

 is chiefly interesting on account of the satisfactory tone 

 of protest against a priori judgments with regard to the 

 principles of dynamics and on account of some attempts 

 which are made to amend the phraseology of the subject, 

 among which "dynamical capacity " figures prominently. 

 It is disfigured by some inaccuracies and obscurities. 



