March 17, 1898] 



NATURE 



461 



extremely variable with us, especially in outlying parts 

 of our islands, where isolation tends to encourage vari- 

 ation. Mr. Barrett's account of the variation of D. con- 

 spersa, Esper, an unusually widely-distributed species 

 with us, is well worthy of the attention of scientific 

 naturalists, and alone furnishes a sufficient reply to any 

 persons who may imagine that our British Lepidoptera 

 may be regarded as worked out. These notes are too 

 long to reproduce in their entirety, as they extend to 

 nearly two pages ; but we may perhaps be allowed to 

 quote a iew sentences relative to the variations exhibited 

 by D. conspersa in the Scottish islands, the entomology 

 of which has only been systematically investigated within 

 the last twenty years or so. 



" It is in the Shetland Isles that the most extraordinary 

 forms are found — the white markings utterly suppressed, 

 or the stigmata alone white, or yellow, or pale ochreous, 

 or black-brown and only indicated by blacker margins ; 

 the subterminal line only indicated by its attendant 

 black clouds, and sometimes the whole surface of the 

 fore-wings smoky-black or olive-black, with but a faint 

 dappling of blacker lines and crescents ; on the other 

 hand, some specimens in these islands are of the form 

 in which the ground colour is intensified, but the mark- 

 ings, though curtailed, sharply white. This last form 

 is also found in the West of Ireland, and a specimen now 

 before me from Sligo is singularly rich in its deep dark 

 colour and snowy blotches. Specimens from the 

 HeJDrides are extremely beautiful, the white markings 

 variegated with yellow and greenish-yellow, or even 

 orange-yellow. Those from Orkney are somewhat 

 similar, or with the white markings small but clear. 

 Although there are so many apparently local strains of 

 variation, all are intimately connected by intermediates, 

 so that, except in some degree in Shetland, it is im- 

 possible definitely to separate the species into varieties. 

 - . . Mr. McArthur, who has collected very extensively 

 in [the Shetland] Islands, assures me that the darkest 

 suffused and nearly unicolorous forms are found mainly 

 on the East coasts, more particularly of the island of 

 Unst, where the rocks among which the food-plant 

 grows are of a very dark colour ; while on the West 

 coast, where the rocks are of a paler colour, the forms 

 found are more nearly normal, with intermediate varieties. 

 Also that in the Hebrides, along with rocks ornamented 

 with yellow lichens, he has found those varieties of the 

 moth having an orange- or olive-yellow tinge. This is 

 the more remarkable in that the moth does not appear 

 there to sit upon the rocks, or to give up its usual habit 

 of hiding in the daytime among herbage." 



Another very interesting species of the same genus is 

 Dianthoecia Barrettit, Doubleday, which was originally 

 captured by Mr. Barrett at the Bailey Lighthouse at the 

 Hill of Howth, near Dublin. It has scarcely been taken 

 anywhere but on this hill, except singly at the Land's 

 End, Ilfracombe, Tenby, Carnarvonshire, and perhaps 

 in the south of Ireland. Mr. Barrett, in common with 

 several other lepidopterists, now regards it as a variety 

 of the continental Dianthcecia luieago, Hiibner, remark- 

 ing : " There is at first sight no resemblance between 

 them, yet, with the exception of the colour, no distinction 

 of any importance can be found." In the larva the 

 spiracles are said to be black in D. Barrettit, and flesh- 

 coloured, encircled with black in D. luteago. 



But before the identity of the two forms can be taken 

 as established, broods of each should be reared in 

 different larva-cages by the same observer, and the 

 NO. 1 48 I, VOL. 5f] 



differences between them carefully noted at each stage. 

 It may be that D. Barrettit is only an incipient species 

 at present ; but it is equally probable that further observ- 

 ation may show that it is abundantly distinct from 

 D. luteago. We incline to think that it is a little pre- 

 mature to class the two forms together, even tentatively, 

 without better evidence. 



We hope that Mr. Barrett will succeed in completing 

 the great work on which he has now made such good 

 progress, and which is likely to remain a most valuable 

 record for all time of the state of our knowledge of the 

 British Lepidoptera at the end of the present century. 

 We cannot speak of the plates, as we have only the 

 smaller edition, without illustrations, before us while 

 writing. W. F. Kirby. 



OUR BOOK SHELF. 



A Suggested Improvement of the Current Theories of 

 the Tides. By J. H. S. Moxly. Pp. 43. (London : 

 Rivingtons, 1898.) 



In this brochure, Mr. Moxly expresses his dissatis- 

 faction with the theory of the tides, as ordinarily 

 accepted, and submits an alternative explanation. 

 Although he opposes the views generally held, he does 

 not exhibit that spirit of antagonism and adopt the 

 language of abuse that too frequently disfigures the 

 writings of those who dissent from authoritative teaching. 

 Where it has been necessary to refer to the papers of 

 Airy or of Darwin, these names are generally mentioned 

 with the respect due to their great reputation, and 

 therefore, though we are as thoroughly opposed to Mr. 

 Moxly as he is to the mathematicians just mentioned, 

 we shall endeavour to adopt the same courtesy towards 

 him that he has shown towards others. 



The author considers that the tides are heaved up by 

 the earth's gravity. The differential attraction of the 

 sun and moon simply gives an opportunity for the 

 earth's gravity to display itself in this manner. This 

 action is illustrated by reference to a football. When 

 the leathern covering is injured, or a seam gives way, the 

 inner india-rubber case bulges out through the opening 

 in the outer cover. " The pressure of the outer case 

 had been removed from one region of the ball, and the 

 pressure of the part which remained did the rest. This, 

 I take it, is exactly how the pressure of the earth's 

 gravity produces the tide." We are all prepared to 

 admit with the author, that the tide-raising force is 

 directly opposed to the action of the earth's gravity, 

 though we might not adopt his phraseology. But 

 another elementary proposition shows that the tide- 

 raising force varies inversely as the cube of the distance 

 of the disturbing body, and we fail to derive this 

 from the football illustration, or from anything directly 

 asserted in the pamphlet, though we notice that some 

 mathematical formulce are quoted in which the third 

 power of the disturbing force is given. Moreover, by 

 another illustration drawn from the observation of 

 pressure applied to a water-bed, the author concludes 

 that the crest of the tidal wave will always be directly 

 under the moon. We understand this to apply to a 

 uniformly ocean-covered earth. From this it is to be 

 gathered that Mr. Moxly does not consider that the 

 angular distance between the tidal crest and the moon 

 is a function of the depth of the ocean, which in the 

 wave theory of the tides it clearly should be. 



But the test of the accuracy of a theory must at last 

 rest in the comparison of its results with those of 

 observation. Mr. Moxly admits this, and therefore a 

 considerable part of the pamphlet is devoted to showing 

 that his theory explains observed facts. Unfortunately, 



