Augtist 23, 1888] 



NA TURE 



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sources from whence he had derived his information, 

 and how far portions of it were based upon his own 

 observations. 



We must take exception to one statement (on p. 79) 

 as rather too sweeping. " Silk-producing Lepidoptera 

 belong exclusively to two families, the Bombyridir and 

 the SaturniidcE." All, or very nearly all, Lepidoptera 

 produce more or less silk ; but even if we understand 

 Mr. Butler to mean "all Lepidoptera which produce silk 

 of economic value," he would still have spoken too posi- 

 tively, for we believe that various species belonging to 

 the Lasiocampidce, and perhaps to other families of 

 Bombyces, have been used as silk-producers in various 

 countries ; as, for example, Libethra cajani in Madagascar. 



Mr. Butler has divided his work into six chapters. The 

 first treats of " The History of Silk Culture," and contains 

 a sketch of the gradual progress of silk-culture and manu- 

 facture, and of the introduction of these industries into 

 one country after another, from their commencement 

 in China, according to tradition, about 2600 B.C., to the 

 present time. One point seems to have been overlooked, 

 viz. the modern origin of the name Morea for the Pelo- 

 ponnesus, and its derivation from the mulberry-tree. 



The second chapter, " The Silkworm : its Form and 

 Life-History," deals with the metamorphoses, and the 

 external structure and changes of the insect in its various 

 stages. The mode of denuding the wings to examine 

 the neuration ; parthenogenesis, and other incidental 

 matters, are likewise noticed. Mr. Butler objects to the 

 term " nervures " as applied to the branching tubes which 

 traverse the wings of butterflies and moths ; but we may 

 be permitted to point out that such terms, when used in 

 a purely technical and conventional manner, though fre- 

 quently incorrect in themselves, rarely mislead anyone. 



Chapter III., "The Silkworm: its Internal Structure," 

 treats, of course, of internal anatomy. Detailed direc- 

 tions are given for dissecting silkworms. The chapter 

 closes with remarks on Lyonnet's great work on the 

 anatomy of the larva of the goat-moth, and with a de- 

 tailed explanation of the position of Bombyx mori in the 

 system of Nature. 



Chapter IV., " The Silkworm : its Rearing and Manage- 

 ment," notices some of the principal races of silkworms, 

 the manner of rearing them, and the mode of preparing 

 the silk. The last paragraph briefly alludes to some 

 allied species of true Bombyx. 



Chapter V. deals with " The Silkworm : its Diseases 

 and Imperfections." The three most serious diseases, 

 flaquerie, muscardine and pebrine, are discussed rather 

 fully, as well as M. Pasteur's method of combating 

 pebrine by microscopic examination of brood females. 



In the concluding chapter (VI.), the author discusses 

 " Wild Silkworms," many of which he figures. His treat- 

 ment of this part of the subject is necessarily somewhat 

 brief, but this is the less to be regretted, as those who wish 

 for further information will probably find much of what 

 they require in Mr. Wardle's " Hand-book of the Collec- 

 tion illustrative of the Wild Silks of India, in the Indian 

 Section of the South Kensington Museum." This book 

 was published by the Science and Art Department of the 

 Committee of Council on Education in 1881 ; and though 

 earlier in date, it will be found a most useful appendix to 

 Mr. Butler's work. 



Mr. Butler himself may fairly be congratulated on his 

 success in compressing so large an amount of useful 

 matter as his book contains into the moderate compass 

 of just 100 pages. W. F. KlRBY. 



OUR BOOK SHELF. 



Allgemeine Geologie. Von Dr. Karl von Fritsch, Professor 

 an der Universitat in Halle. (Stuttgart : J. Engelhorn, 

 1888.) 



This is one of a very useful series of volumes which is 

 appearing under the editorship of Dr. Friedrich Ratzel, 

 with the title of " Library of Geographical Handbooks." 

 As the subjects of glaciers and of volcanoes and earth- 

 quakes have had special volumes of the series devoted to 

 their discussion, while many other problems of geological 

 interest are treated of in separate monographs, such as 

 those which deal with the geography of the ocean, and 

 the morphology of the earth's surface, Dr. von Fritsch has 

 been able to limit the scope of the work now before us to 

 certain definite lines of inquiry. The first division of the 

 book is devoted to "Geophysiography," or a discussion of the 

 features of the earth as a member of the solar system, and 

 of the relations of the atmosphere and ocean to the litho- 

 sphere or solid crust of the globe. The second division, 

 " Geotektonik," deals with the forms and relations of the 

 rock-masses that build up the solid crust, and is treated with 

 considerable fullness, the illustrations being for the most 

 part new, and not of the kind which find a place in the 

 ordinary text-books of geology. In the third part, " Geo- 

 chemistry," or chemical geology, we have a short sketch 

 of the present state of petrography, or the description of 

 rocks, followed by remarks on petrogeny, or the theory of 

 their origin. It would be unfair to expect, in the 175 

 pages at the author's disposal, anything like a complete 

 treatment of the numerous and difficult problems presented 

 by petrological science at the present day, but it is certainly 

 possible to conceive of a bolder and more masterly treat- 

 ment of the whole question than is found in the present 

 work. " Geomechanik," or physical geology, treats of the 

 questions usually grouped by English writers under the 

 head of dynamical geology ; and the fifth and concluding 

 portion of the work is devoted to " Geogenie," or a general 

 sketch of historical geology. The work is of interest to 

 English students and teachers of geological science, as 

 illustrating the general methods of treatment of the subject 

 which prevail in Germany. Without aiming at the com- 

 prehensive character which belongs to the well-known 

 treatises of Credner and Giimbel, this book forms an 

 admirable sketch of the chief facts and theories of 

 geological science, which are presented always in an 

 attractive and sometimes in a somewhat novel manner. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take 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 communis 

 cat ions. ~\ 



Functionless Organs. 



In an interesting letter which appeared in Nature 

 (p. 341), under the above title, the Duke of Argyll brings 

 forward a "doctrine of prophetic germs" as explanatory of 

 certain rudimentary structures. He refers particularly to the 

 electrical organ of the skate, which he regards as an example of 

 such a germ. The doctrine is that these functionless organs are 

 not structures which have been useful, but are endowed with 

 " utilities yet to be." 



In the lecture which I gave at the Royal Institution, on 

 " Electrical Fishes," in May 1887, I pointed out, in discussing 

 the particular instance referred to, that the difficulty suggested 



