February 12, 1903] 



NA TURE 



OtJ 



seventeen species are referred to the first three sub- 

 families altogether, the whole of the remainder falling 

 under the fourth, and typical, subfamily. 



The sexes are very different, and it is not always easy 

 to identify them, the males being winged, and often 

 much smaller and slenderer than the females. The 

 head, thorax and abdomen are usually sharply separated, 

 and the body is clothed with very thick down, and is 

 more or less brightly coloured, for even where the 

 prevailing colour of the abdomen is black, it is usually 

 marked with bands or large spots of red, yellow or 

 silvery white. 



So far as their habits have yet been observed, the 

 Mutillida? are parasitic in the nests of various ground- 

 bees and burrowing wasps. 



We have so recently reviewed one of the volumes of 

 this series that it is, perhaps, unnecessary to say more 

 than that the arrangement of this volume is similar to 

 that of its predecessors and that it appears to be fully 

 equal to them in execution, both as regards the text and 

 plates. W. F. K. 



Publications of West Hendon House Observatory, Sun- 

 derland. No. 2. By T. W. Backhouse, F.R.A.S. 

 Pp. viii + 161. (Sunderland: Hills and Co., 1902.) 



This volume contains the detailed observations which 

 have been made by Mr. Backhouse on the structure of 

 the sidereal universe, comets Barnard (1886) and Holmes 

 (1892), the Zodiacal Light, the Aurora Borealis, and vari- 

 able and suspected variable stars. 



The first part of the observations of the sidereal uni- 

 verse was contained in a previous similar publication 

 (No. 1), and in this second part the author deals with the 

 observations of radiating systems, lines and parallelisms 

 amongst the stars, and the Milky Way. 



The author has arranged his table of observations of 

 "Auroras" (which extend from January, i860, to Mid- 

 summer, 1896) so as to indicate whether or not there is 

 any foundation for supposing the appearances of this 

 phenomenon to have a periodic fluctuation. The table, 

 together with the accompanying curve, indicates a period 

 of sixty-five days, which includes a well-marked succession 

 of maxima at intervals of twenty-eight days. 



The last section of the book, dealing with the observ- 

 ations of variable and suspected variable stars, includes 

 an introduction on the " Calculation of Star Magnitudes," 

 observations of the " Orange Stars near >; Geminorum " 

 and the "Brighter Stars in Hercules and Neighbour- 

 hood," together with a descriptive diagram of the varia- 

 tion of V Aquilae. 



Buttermaking on the Farm and at the Creamery. By 

 C. W. W. Tisdale and T. R. Robinson. (London : 

 John North, the Dairy World Office, 1903.) Price is. 

 This little book is, strictly speaking, a handbook on 

 practical buttermaking. It has the merit of being 

 thoroughly up-to-date, in that the whole process of 

 buttermaking is dealt with in minute detail, and the 

 practice recommended is based on the latest scientific 

 research connected with dairying. It does not de- 

 scribe dairy implements or breeds of cattle, but simply 

 the making of butter and the management of the 

 milk and cream from which it is produced, and it is 

 probably the best of the handbooks on practical butter- 

 making. The treatment of milk and cream at the 

 factory is fully dealt with, as well as at the farm, and 

 also such subjects as pasteurisation, ripening of 

 cream on a large scale, purchase of milk according 

 to quality, and the packing and marketing of butter. 

 There are also one or two excellent illustrations, show- 

 ing the appearance of butter in different stages of 

 churning and making. Douglas A. Gilchrist. 



NO. 1737, VOL. 67] 



LETTERS TO THE EDITOR. 



{The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he utiderlake 

 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. ] 



Sir Edward Fry on Natural Selection. 



I have only just read the memoir of Sir Edward Fry in the Jan- 

 uary number of the Monthly Review on "The Age of the Inhabited 

 World." Withits general purport I am in sympathy, and I rejoice 

 in the opportunity of offering a tribute of praise to the extreme 

 lucidity of the language in which it is expressed ; but for those 

 very reasons I desire to protest against one of his arguments, 

 which seems to me so faulty as to seriously compromise the 

 value of the memoir as a whole. He is endeavouring to show 

 that natural selection is incapable of doing much that has been 

 accredited to its agency, and uses, p. 7S, these words in respect 

 to mimetic insects : — 



". . . . the useful deception will not take place until the pro- 

 tected form is nearly approached. Thus during the whole 

 interval occupied in passing from the normal form of group A to 

 near the normal form of group B, natural selection will have 

 been entirely inoperative. . . . Either birds are deceived by a 

 small amount of imitation or they are not. If they are, natural 

 selection cannot have produced perfect imitation ; if they are 

 not so deceived, then group A has passed over from its original 

 form to something close upon the form of group B without any 

 guidance from this principle." 



I deny this sharp dilemma and assert the existence of many 

 intermediate stages. Two objects that are somewhat alike will 

 be occasionally mistaken for one another when the conditions 

 under which they are viewed are unfavourable to distinction. 

 The light may be faint, only a glimpse of them may have been 

 obtained, the surroundings may confuse their outlines. While 

 these conditions remain unchanged, the frequency of mistake 

 serves as a delicate measure of even the faintest similarity. 

 Prof. McKeen Cattell measured in this way the relative resem- 

 blances (in other words the want of distinction) between various 

 printed letters of the alphabet. He placed them on a screen 

 behind a drop slide that had a horizontal slit, giving a uniformly 

 brief glimpse of the letters while the slide was falling. He 

 found, as might have been expected, that "i" was often mis- 

 taken for " 1," " k " for " h," and so on, each couplet with its 

 own special degree of frequency, which gave a numerical 

 measure of the relative resemblances of the letters. Many other 

 letters that seem ordinarily very unlike were occasionally mis- 

 taken for one another, each in a definite percentage of cases. 

 So it must be with insects. If one of the edible group A has 

 individual peculiarities within the limits of variation, that give 

 it a resemblance, however slight, to one of the noxious group 

 B, it will occasionally be mistaken by a bird for a B and 

 allowed to live unharmed. The similarity may be due to a 

 characteristic attitude, to a blotch of colour, to a preference for 

 resting on a part of the foliage to which its own form bears 

 some likeness, or to other causes. In any case, it may well 

 prove to be the salvation of 1, 2 or more per cent, of those 

 that would otherwise have been seen and eaten. If so, the 

 thin edge of natural selection will have found an entrance, and 

 its well understood effects must follow. Francis Galton. 



Hotel Europe, Rome. 



The Principle of Least Action. 



Mr. Heaviside has done good service in calling atten- 

 tion to the misuse of this principle ; and certain theories of 

 electromagnetism, which have been recently proposed, afford 

 a striking illustration of the value of his remarks and the 

 limits within which the legitimate application of the prin- 

 ciple is confined. 



In many branches of physics, the equations of motion and 

 the boundary conditions of the dynamical system under 

 consideration cannot be obtained without making some 

 hypothesis, which may or may not be true. One method of 

 testing the truth of the hypothesis is by appeal to experi- 

 ment, but the legitimate use of the P. of L. A. frequently 

 supplies another. For the original hypothesis, when ex- 

 pressed in terms of mathematical symbols, leads to an 

 energy function, from which the equations of motion and 



