May 20, 1909] 



NATURE 



developed singer of all, this quality can be found as 

 well as rhythm and invention. Whether we agree with 

 him or not in some of these details, it is a pleasure to 

 be able to say with confidence that all he writes 

 deserves careful study, for which every conscientious 

 ornithologist will be the better. 



The only thing that seems wanting is a discussion 

 of the quality of tone (not quantity) in various 

 species. Thus the formal likeness between the songs 

 of the chaffinch and the willow-wren is noticed (p. 

 31), but nothing is said of the fact that they are 

 produced by totally different instruments. To the 

 ear of the present writer the songs of both species 

 of redstart are " played " on an instrument which 

 no other bird possesses. We would suggest that Dr. 

 Hoffmann sjiould add a section on this subject in 

 another edition, and shorten, if need be, the discussion 

 at the end of the volume on the use made by musical 

 composers of the songs of birds, which is only of 

 incidental interest. Before leaving this interesting 

 work, which well merits translation into English, it 

 may be as well to say that the author is disposed to 

 reject Darwin's theory of the development of song 

 by se.xual selection, and to hold that the root of it 

 is to be found in the enjoyment of life and the love 

 of play, especially, but not entirely, in the breeding 

 season. W. W. F. 



OUR BOOK SHELF. 



The Scientific Aspects of Luther Burhank's Work. 



By D. S. Jordan and \'. L. Kellogg. Pp. xiv+115. 



(San Francisco : A. M. Robertson, 1909.) Price 



2 dollars net. 

 This is a small book, consisting of two papers re. 

 printed from the Popular Science Monthly, describing 

 and appreciating the work of the great American 

 plant-breeder. It is attractively illustrated by 

 photographs, and is intended for the general as well 

 as the scientific reader. 



Luther Burbank was born in 1849, and after a 

 local education started life in his uncle's plough 

 factory. He soon gave this up for market garden- 

 ing, and in 1875 moved to Santa Rosa, California, 

 where he has since worked on a large scale, and 

 produced many new and important varieties, both of 

 fruits and flowers. He has discovered no new laws, 

 but his results are so obviously successful that it is 

 interesting to know the methods by which they have 

 been obtained. Like most practical men, he is a firm 

 believer in the heritability of the direct effects of 

 environment, but he makes most use of the indirect 

 ones — the " indefinite variations " of Darwin — and 

 recognises as their chief cause the re-combination of 

 characters consequent on hybridisation, and, in a lesser 

 degree, on cross- or self-fertilisation. 



The first step in the method usually followed is the 

 inducing of these variations by nutritive changes or 

 by the crossing of forms as widely separated as is 

 compatible with fertility. The useful variations are 

 then accumulated by stringent selection until they 

 become fixed. Mr. i3urbank finds that six genera- 

 tions are generally sufficient to accomplish the pro- 

 cess. He holds that there is practically no limit to 

 the results which can be obtained by unassisted selec- 

 tion, and many of his size and colour varieties of 

 flowers have been obtained by this method alone. 

 Sometimes, on the other hand, a new variety is pro- 

 duced by the careful propagation, without much 

 NO. 2064, "VOL. 80] 



selection, of one individual which showed a fortunate 

 mutation. The Burbank stoneless plum is an ex- 

 ample of the effective combination of the three pro- 

 cesses of searching for natural mutations, hybridising, 

 and selection. A plum was found in a small wild 

 species with only part of a stone. This species was 

 crossed with the French prune, and some of the 

 offspring found to be quite stoneless. Further selec- 

 tion is still increasing the proportion of stoneless, and 

 at the same time large, fruits. The desirable qualities 

 of two varieties can generally be combined by cross- 

 ing ;_ indeed, some of the offspring often possess a 

 quality in a higher degree than either of their parents. 

 Some of the photographs illustrating the increase of 

 size in fruits show this in a striking manner. 



We do not for a moment doubt that Mr. Burbank 

 has "a broad intelligence and a sensitive soul." If 

 he is also " as sweet, straightforward, and as un- 

 spoiled as a child," it is just possible that he can 

 stand being told so. But his portrait is so singularly 

 charming that it might have been left to speak for 

 itself. 



Text-book of Petrology, containing a Summary of 

 the Modern Theories of Petrogenesis, a Descrijytion 

 of the Rock-forming Minerals, and a Synopsis of the 

 Chief Types of the Igneous Rocks and their Distri- 

 bution, as illustrated by the British Isles. By Dr. 

 F. H. Hatch. Fifth edition, revised and re- 

 written. Pp. xvi+404. (London : Swan Sonnen- 

 schein and Co., Ltd., 1909.) Price 7s. 6d. net. 

 This new edition of a well-known text-book for 

 students marks a decided advance on its predecessors. 

 It is clearly written, well illustrated, and has, as a 

 rule, been brought up to date. 



There is a brief but readable account of the eutectic 

 theory of the process of crystallisation of igneous rocks, 

 as well as of the different explanations which have 

 been offered of the formation of porphyritic crystals. 



The descriptions of the rock-forming minerals are 

 in most respects accurate and sufficient. The author 

 disclaims any intention of dealing with the optical 

 determination of minerals, but as he makes use of 

 the interference colours for the purpose of estimating 

 the birefringence, he might with advantage have gone 

 a little further and shown how easily an approximate 

 quantitative determination of the relative retardation 

 and birefringence may be made. Such expressions as 

 "weak," "moderate," "very strong," "polarising 

 in grey tints," "brilliant chromatic polarisation," 

 though commonly employed, have very little scientific 

 value, especially when the variation in thickness of 

 rock-slices, even by good makers, is considered. In 

 the same way, if the angle of extinction be employed 

 for determinative purposes, the student should be 

 taught to discriminate between the positive (slow) and 

 negative (fast) directions of extinction. The statement 

 that " between crossed nicols the rhombic pyroxenes 

 extinguish of course straight " is too sweeping. Cer- 

 tain directions of section show quite an appreciable 

 angle of extinction. 



The author adopts analytical formulae for the rock- 

 forming minerals, a procedure which is justified bv 

 the clearness with which the composition is indi- 

 cated and the ease with which it is remembered, but it 

 may be noted that the abbreviation " Ab " for albite 

 represents, not Na2O.ALO3.6SiO: as stated, but half 

 that formula. 



The primary classification of igneous rocks into 

 plutonic, hypabyssal and volcanic, which is adopted, 

 is sanctioned by almost universal usage, though it is 

 as unreasonable as a fundamental division of the 

 vegetable kingdom into roots, stems, and leaves. Each 

 class of rocks is separated into families and types, 



