Jlay 12, 1881] 



NATURE 



27 



wave propagated through it. If under these particular 

 circumstances the quantities — 



dy d 2' ds ax d.v dy 

 are given at each point, these three quantities are in general 

 sufficient to define the direction of the normal to the wave 

 front and the direction of disturbance. Hence — and here is 

 the fallacy — xhege?ieral expression for V must be a function 

 of these three quantities. MacCuIlagh was probably 

 unaware that these three quantities simply defined the 

 angles through which the element about the point .r_)'^ 

 was rotated by the displacement. That his expression 

 for the function Fnot only rests on wrong reasoning but 

 is actually wrong, for the case in question, where V is 

 supposed to depend upon t/ie change of form of the 

 elementary parallelepiped, is easily seen. Suppose the 

 deformation to be irrotational ; through a given space on 

 MacCuUagh's theory K vanishes : but vvc know it may be 

 anything we please. Again, suppose the medium within 

 a given space to be simply turned through a given angle 

 without change of shape : MacCuIlagh gives a finite value 

 to V : but we know that in this case V is zero. It is a 

 pity that MacCuIlagh did not keep to the method of his 

 papers on metallic reflection and on transmission in 

 quartz, and simply say, without admitting that V depends 

 on the change of form of the elementary parallelepiped, 

 "let us assume that K is a quadratic function of 



( — — — ), &c." His theory would then have had a 

 \dy dsP ^ 



certain amount of analytical similarity with Maxwell's 

 theory of electromagnetic propagation of light, though 

 giving not the slightest adumbration of the physical basis 

 of that theory, the facts which it covers being almost 

 unknown when MacCuIlagh wrote. MacCuUagh's expres- 

 sion for V as a theory regarding the ether as an elastic 

 solid is misleading, but we are certainly not compelled 

 to be so materialistic. To the student of physical optics 

 we should say, first read Green, then read and criticise 

 MacCuIlagh and Cauchy, and finally read Maxwell, not 

 once or twice only, but until you understand him. 



067? BOOK SHELF 

 On tlte Structure and Affinities of the Genus Monticuli- 

 pora and its Sub-genera. By H. AUeyne Nicholson, 

 M.D., D.Sc, &c., Professor of Natural History in the 

 University of St. Andrews. Pp. 240, and vi. Plates. 

 (Edinburgh and London : Blackwood and Sons, 1S81.) 



This is a most elaborate work, on one of the most puzzling 

 groups of palaeozoic fossils, by the accomplished and indus- 

 trious Professor of Natural H istory at St. Andrews. He gives 

 the general history and literature of the genus, describes 

 the morphology, dealing carefully with the dimorphism of 

 the corallum, treats of the development of the forms, and 

 compares them with Heteropora amongst the Bryozoa. 

 Then the affinities of Chaetetes and Stenopora are con- 

 sidered, and those of the Helioporidse also. A chapter is 

 devoted to the sub-divisions of the genus and to the con- 

 sideration of the propriety of separating from it Fistuli- 

 pora, Coustellaria, and Dekayia. Finally five chapters 

 are occupied by the consideration of as many sub-genera. 

 Yet the author modestly says that it is not a monograph 

 of the Monticuliporidae ! The book is particularly valuable 

 on account of the mass of careful description it contains, 

 and the plates and cuts are excellent, and everybody who 

 has tried to make out these tubular fossils will be grateful 



to Prof. Nicholson for his work. Like most paleontolo- 

 gists, he has suffered from the fact that his predecessors 

 have described genera and species from very imperfect 

 specimens. This is the curse of modern palaeontology, 

 and a clean sweep should be made of every classification 

 which is not clear and definite, and which was founded 

 on bad specimens. The difficulty of the subject taken up 

 by the author may be appreciated by noticing the 

 synonymy of the species ; and it is interesting to notice 

 how recent investigations by Busk, Waters, and Moseley 

 are influencing the palaeontology of very remote ages. 



The author states that in Monticulipora there are no 

 septa, and the walls are imperforate, whilst in Hctcropitra 

 the walls are traversed by a very remarkable and excep- 

 tionally developed canal system ; hence he separates the 

 groups, but states — " In the face of the above distinctions 

 I feel compelled to believe, in the meanwhile, that there 

 is no real relationship at all between Heteropora and 

 Moniieulipora." " On the other hand there are strong 

 resemblances between Monticiilipora and its allies and 

 various undoubted corals — principally perhaps the Helio- 

 poridce " " I am at present disposed to regard the Monii- 

 culiporidee as ancient groups of the Atcyonaria!^ 



P. M. D. 



The Evolutionist at Large. By Grant .\llen. (London : 



Chatto and Windus, iSSi.) 

 Wider and wider grows the field over which news- 

 papers and magazines exert their distributive influence. 

 Verily, they sow beside all waters, and great is the 

 variety of the seed. Their readers find a royal road to 

 learning the contents of books which they are too hurried 

 to read in full, in short essays which collect the c>sencc, 

 omit the difficulties, and state the conclusions of the 

 writers in the clearest and most unqualified terms. It is 

 satisfactory to find that an effort is made to supply modern 

 science to such readers from competent pens. 



Mr. Grant Allen has collected into this engag;ing little 

 volume a series of well-judged attempts to perform thi^ 

 which have appeared in the St. James's Gazette, and no 

 reader who would consult that class of publication for 

 scientific ideas could help being interested and, we should 

 hope, led on to further inquiries by it. Mr. Allen describes 

 himself fairly when he says (p. 109;, "lam not a butterfly- 

 hunter myself. I have not the heart to drive pins through 

 the pretty creatures' downy bodies, or to stifle them with 

 reeking chemicals ; though I recognise the necessity for 

 a hardened class who will perform that useful office on 

 behalf of science and society, just as I recognise the 

 necessity for slaughtermen and knackers. But I prefer, 

 personally, to lie on the ground at my ease and learn as 

 much about the insect nature as I can discover from 

 simple inspection of the living subject as it flits airily from 

 bunch to bunch of bright-coloured flovvers." And any one 

 who sympathises with such feelings will delight in the 

 company of " The Evolutionist at Large." 



Nearly all the fresh lights which have been thrown upon 

 the relations of the natural world by the teachings of 

 Darwin and Herbert Spencer are here condensed and 

 exhibited in the most simple gossipping style : while it is 

 hardly necessary to say that the most puzzling questions 

 that remain unanswered will suggest themselves on many 

 a page of such an author's book. His disquisitions on the 

 extent of animal feeling (p. 50), upon the origin of two 

 eyes and the cross-connection of them and other organs 

 with the brain (p. 103), are very interesting. But the 

 most striking question which time after time turns up, as 

 we might expect in such a book by Mr. Grant Allen, is 

 the origin of our sesthetic sense : and since the sense of 

 beauty is little else than a feeling of harmony with and 

 admiration for the forms, colours, and adornments of 

 flowers and animals ; and as these are all the result of 

 their selection by animals in choosing their haunts and 

 their mates, we are landed at the rather humiliating con- 



