400 



NATURE 



[January 26, 191 1 



qualitative one. Fluctuating variations may be (and 

 sometimes are) very considerable, while mutational 

 changes may be extremely small. The difference be- 

 tween them may perhaps be best appreciated by say- 

 ing that a fluctuating variation is the outcome of 

 a changed environment on an otherwise unchanged 

 mechanism, whilst a mutation is the result of a 

 changed internal mechanism, and even with a con- 

 stant environment the product will not be identical 

 with that of the unchanged type reacting with a 

 similar environment. It is the change of the vital 

 machinery which necessarily will shift the metabolism 

 of the organism into a new channel, and henceforth 

 will produce a new form, stable, until once more the 

 constitution, or chemico-mechanical framework of the 

 race, undergoes further modification. The change 

 itself may be small or it may be large, but it is 

 essentially in its occurrence at all, and independently 

 of its magnitude, that the production of a mutation 

 depends. Furthermore, that to this new constitution 

 is owing the circumstances that mutants are on such 

 a different plane from fluctuating varieties so far as 

 reversions are concerned. It may be argued that this 

 smacks rather of hypothetical statement than of proved 

 explanation of the facts, but it may be urged that 

 fluctuating variations and mutations at any rate do 

 express distinct kinds of variations, that these are 

 not merely quantitatively different, and that it is 

 therefore probable that they depend on the existence 

 of different factors, in the two categories. 



The second portion of the volume deals with plant 

 physiology. It is written in the incisive style we 

 have been accustomed to expect from the late Prof. 

 Barnes; the arrangement of the material is good, 

 and the mode of presentation appears to us to be 

 very well suited to the requirements of those classes 

 of students for whom it is designed. A cautious 

 attitude which is much to be commended on general 

 grounds is observed towards many "explanations" of 

 physiological phenomena. Many interesting data not 

 commonly met with in works of this kind are in- 

 cluded, and render the book valuable to student and 

 teacher alike. 



It will be apparent from the foregoing that we 

 expect the " Chicago Text-book " to take its place 

 as a valuable addition to the class books of botany, 

 and we hope the appearance of the concluding part 

 may not be long delayed. J. B. F. 



PRACTICAL ZOOLOGY. 

 Leitfaden fiir das zoologische Praktikiim. By Prof. 

 Willy Kiikenthal. Funfte umgearbeitete Auflage. 

 Pp. viii + 320. (Jena: Gustav Fischer, 1910.) 

 Price 7 marks. 



IT is a significant fact that no British zoologist 

 has yet thought it worth while to write a text- 

 book of practical zoology on the lines of Prof. 

 Kiikenthal's admirable work, which has now reached 

 its fifth edition. The reason is perhaps to be found 

 in the fact that zoology is so very lightly esteemed 

 by those who have the ordering of our educational 

 system. For this no doubt zoologists themselves are 

 largely to blame. The specialisation of original re- 

 'search during the last twenty years has led to the 

 NO. 2152, VOL. 85] 



accumulation of an enormous number of facts, which, 

 though valuable and interesting in themselves, are 

 from the educational point of view to a very large 

 extent redundant. 



The student is expected to familiarise himself with 

 a vast mass of minute morphological, embryological, 

 and systematic details, as well as with a great deal 

 of more or less speculative matter, much of which 

 has not yet stood the test of time. He can scarcely 

 see the wood for the trees, and realises that the 

 subject has become one of the most difficult, if not 

 quite the most difficult, which he can take up for 

 examination purposes. At the same time, the almost 

 complete absence of zoology from our school curricula 

 renders the subject comparatively useless from the 

 point of view of the student who is qualifying him- 

 self as a teacher. In Germany the study of zoology 

 appears to be much better appreciated, and this is 

 probably largely due to the fact that teachers treai 

 it more reasonably and do not expect their students 

 to accomplish an impossible task. 



The work before us affords an excellent survey 

 of the animal kingdom from the laboratory point ot 

 view. It is divided into twenty "Kurse," each deal- 

 ing with a special group of animals. We do not 

 know how long each " Kursus " is supposed to 

 occupy, but the subject-matter dealt with in each 

 would in this country be regarded as far too much 

 for a single practical class. Thus the frog, the 

 pigeon, the lizard, and the rabbit are each dealt with 

 in a single "Kursus," and so are no fewer thar 

 thirteen types of Protozoa. Each "Kursus" consist- 

 of technical instructions, a general review of the 

 group or groups dealt with, and a special description 

 of selected types. 



The plan of the work is very well carried out, an ' 

 the numerous illustrations are excellent. Students o 

 Marshall's "Frog," or Marshall and Hurst's Zoology 

 would no doubt regard the treatment of types as ver 

 superficial, but it is at any rate an open question 

 whether it is not more important to gain a really 

 comprehensive first-hand knowledge of the animal 

 kingdom than to attempt to deal with a very small 

 number of types in great detail. It must be 

 borne in mind that Prof. Kukenthal's book is appar- 

 ently intended for students of " Hochschulen^" wli 

 are only taking a single year's work in zoology. Fui 

 those who are able to take two or three years we do 

 not doubt that the mode of treatment adopted in 

 the English text-books above named would be prefer- 

 able for the first year, but a work such as that under 

 review, sufficiently amplified, is badly wanted for 

 more advanced students in this country. A. D. 



lONISATION OF GASES BY COLLISION. 

 The Theory of lonisation of Gases by Collision. By 

 Prof. John S. Townsend, F.R.S. Pp. xi-h88. 

 (London : Constable and Co., Ltd., 1910.) Pri 

 35. 6d. net. 



IN various papers published during the last ten 

 years Prof. Townsend has developed a theory of 

 the ionisation of gases by collision, and has published 

 experimental results which give it strong confirma- 



