486 



NATURE 



[March 22, 1888 



ment is very detailed for an elementary book, but there 

 is nothing beyond the capacity of those for whom it is 

 intended. The author is of opinion — and we quite agree 

 with him — that meagre accounts lead to inaccurate ideas, 

 inasmuch as they are not of sufficiently general applica- 

 tion. As far as desirable, and in accordance with the 

 syllabus, simple experiments have been introduced. The 

 main results of the Challenger Expedition are also 

 explained, and illustrated by diagrams. 



The astronomical portion leaves nothing to be desired. 



In addition to 150 excellent diagrams, there are ten 

 maps, illustrating the distribution of temperature and 

 pressure, volcanoes and earthquakes, &c. The diagram 

 of the geological formations shows the general physical 

 appearance of the strata, along with the characteristic 

 fossils of each. 



The book is beautifully printed, and is sure to win the 

 favour of all who use it^ whether as students or teachers. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he under- 

 take to return, or to correspond with the -writers of, 

 rejected manuscripts intended for this or atiy other part 

 of Nature. No notice is taken of anonymous communi- 

 cations.'] 



Dr. Whewell on the Origin of Species. 



In his essay on the " Reception of the ' Origin of Species,' " 

 Prof. Huxley writes : — 



"It is interesting to observe that the possibility of a fifth 

 alternative, in addition to the (our he has stated, has not dawned 

 upon Dr. Whewell's mind " (" Life and Letters of Charles 

 Darwin," vol. ii, p. 195). 



And again, in the article "Science," sujoplied to "The 

 Reign of Queen Victoria," he says ; — 



" Whewell had not the slightest suspicion of Darwin's main 

 theorem, even as a logical possibility " (p. 365). 



Now, although it is true that no indication of such a " logical 

 possibility" is to be met with in the " History of the Inductive 

 Science?," there are several passages in the Bridgewater Treatise 

 which show a glimmering idea of such a possibility. Of these 

 the following are, perhaps, worth quoting. Speaking of the 

 adaptation of the period of flowering to the length of a year, 

 he says : — 



" Now, such an adjustment must surely be accepted as a proof 

 of design, exercised in the formation of the world. Why 

 should the solar year be so long and no longer ? or, this being 

 such a length, why should the vegetable cycle be exactly of the 

 same length ? Can this be chance ? . . . And, if not by chance, 

 how otherwise could such a coincidence occur than by an inten- 

 tional adjustment of these two things to one another ; by a 

 selection of such an organization in plants as would fit them to 

 the earth on ^^ hich they were to grow ; by an adaptation of 

 construction to conditions ; of the scale of construction to the 

 scale of conditions? It cannot be accepted as an explanation 

 of this fact in the economy of plants, that it is necessary to 

 their existence ; that no plants could possibly have subsisted, 

 and come down to us, except those which were thus suited to 

 their place on the earth. This is true ; but it does not at all 

 remove the necessity of recurring to design as the origin of the 

 construction by which the existence and continuance of plants 

 is made possible. A watch could not go unless there were th e 

 most exact adjustment in the forms and positions of its wheels ; 

 yet no one would accept it as an explanation of the origin of 

 such forms and positions, that the watch would not go if these 

 were other than they were. If the objector were to suppose 

 that plants were originally fitted to year- of various lengths, and 

 that such only have survived to the present time as had a cycle 

 of a length equal to our present year, or one which could be 

 accommodated to it, we should reply that the assumption is 

 too gratuitous and extravagant to require much consideration." 

 Again, with regard to " the diurnal period," he adds : — 

 " Any supposition that the astronomical cycle has occasioned 

 the physiological one, that the structure of plants has been 

 brought to be what it is by the action of external causes, or that 



such plants as could not accommodate themselves to the existing 

 day have perished, would be not only an arbitrary and baseless 

 assumption, but, moreover, useless for the purposes of ex- 

 planation which it professes, as we have noticed of a similar 

 supposition with respect to the annual cycle." 



Of course, these passages in no way make against Mr. Hux- 

 ley's allusions to Dr. Whewell's writings in proof that, until 

 the publicatioii of the "Origin of Species," the "main 

 theorem "of this work had not dawned on any other mind, save 

 tha!. of Mr. Wallace. But these passages show, even more empha- 

 tically than total silence with regard to the principle of survival 

 could have done, the real distance which at that time separated 

 the minds of thinking men from all that was wrapped up in 

 this principle. For they show that Dr. Whewell, even after he 

 had obtained a glimpse of the principle " as a logical possi- 

 bility," only saw in it an " arbitrary and baseless assumption." 

 Moreover, the passages show a remarkable juxtaposition of the 

 very terms in which the theory of natural selection was after- 

 wards formulated. Indeed, if we strike out the one word 

 "intentional" (which conveys the preconceived idea of the 

 writer, and thus prevented him from doing justice to any 

 naturalistic view), all the following parts of the above quota- 

 tions might be supposed to have been written by any Darwinian. 

 "If not by chance, how otherwise could such a coincidence 

 occur, than by an adjustment of these two things to one an- 

 other; by a selection of such an organization in plants as would 

 fit them to the earth on which they were to grow ; by an adapt- 

 ation of construction to conditions ; of the scale of construction 

 to the scale of conditions ? " Yet he imme.liately goes on to 

 say : "If the objector were to suppose that plants were origin- 

 ally//'/^'^/ to years of various lengths, and that such only have 

 sni-vived to the present time ... as could be accommodated to 

 it {i.e. the actual cycle), we should reply that the assumption 

 is too gratuitous and extravagant to require much considera- 

 tion." Was there ever a more curious exhibition of failure to 

 perceive the importance of a "logical possibility" ? and this at 

 the very time when another mind was bestowing twenty years 

 of labour on its "consideration." George J. Romanes. 



The Fog Bow. 



The complete theory of the rainbow, as developed by Sir 

 George Airy (Camb. Phil. Trans., vi. p. 379, 1836), besides 

 explaining the supernumerary bows, shows that the main bow 

 has a radius somewhat smaller than that calculated on the 

 ordinary geometrical theory. The smaller the drops the greater 

 is the discrepancy. With the tiny drops composing a fog, the 

 discrepancy is so marked that the bow receives a new name — 

 the fog-bow, or " arc-en-ciel blanc." Mr. Mohn's (Nature, 

 February 23, p. 391) nearly simultaneous measurements of the 

 fog- bow and Ulloa's rings afo.d a capital opportunity of putting 

 the theory to the test, for from the latter phenomenon we can 

 readily calculate the average size of the particles. 



Not having Airy's paper within reach, I have had to be con- 

 tent with the incomplete account given by Verdet ("Le9ons 

 d'Optique Physique," tom. i. p. 414). " Assuming ^= i '333, I 

 find for the angular discrepancy — 



)3 = 0-467 7>l(h 



where K is the wave-length, a the radms of the drop, and m is 

 determined by the condition that the integral — 



/ cos — (7f^ — mT.v)d%v 

 Jo 2 



should be a maximum. This integral was calculated by Airy 

 for a series of values of m, but Verdet does not quote the results. 

 Some rough approximations lead me to the conclusion that m 

 lies between I'o and i'3, and very much nearer the latter. 

 For the radius of the first Ulloa's ring we have 



a = 0'82\/a. 



Mr. Mohn measured this radius as 1° 31'. Using this value, 

 and taking m as i -25, I find j8 is the circular measure of 3° 24'. 

 The geometrical theory gives the radius of the rainbow 42° 2'. 

 So in this particular case the fog bow should have had the 

 radius 38° 38'. Mr. Mohn gives two measurements, taken 



