Feb. 2 2, 1872J 



NA TURE 



319 



that in the Pleuronectidre, or Flat-fish, the eyes are oppo- 

 site in the young state, and afterwards become placed 

 both on the upper side of the head— that this change 

 must have taken place suddenly, since any small ap- 

 proach to it would not be useful — is met by an ingenious 

 argument, previously advanced by Malm. It is stated 

 that " the Pleuronectida:, whilst still very young and still 

 symmetrical, with their eyes standing on opposite sides 

 of the head, cannot long retain a vertical position, owing 

 to the excessive depth of their bodies, the small size of 

 their lateral fins, and to their being destitute of a swim- 

 bladder. Hence, soon growing tired, they fall to the 

 bottom on one side. While thus at rest, they often twist, 

 as Malm observed, the lower eye upwards to see above 

 them, and they do this so vigorously that the eye is 

 pressed hard against the upper part of the orbit. The 

 forehead between the eyes consequently becomes, as could 

 be plainly seen, temporarily contracted in breadth. On 

 one occasion Malm saw a young fish raise and depress the 

 lower eye through an angular distance of about 70^." 



The objections urged by Niigeli in his "Begriff und 

 Entstehung der naturhistorischen Art," with respect to 

 plants, that the families of plants differ chiefly from each 

 other in morphological characters, which appear to be 

 quite unimportant to the welfare of the speeics, are com- 

 bated on the ground that we ought to be exceedingly 

 cautious in pretending to decide what structures now are 

 or have formerly been of use to each species. While 

 admitting that in earlier editions he underrated the fre- 

 quencyand importance of modifications due to spontaneous 

 variability, Mr. Darwin points out that many peculiarities 

 of structure, lately supposed to be simply morphological, 

 are now known to be intimately connected with facilities 

 for fertilisation. 



On the whole it seems to us that each succeeding edition 

 of the " Origin of Species " lessens the distance between 

 Mr. Darwin and those who believe that the influence 

 of natural selection, though a vera causa, has been 

 overrated as an element in the evolution of species. If it is 

 admitted that important modifications are due to " spon- 

 taneous variability," that natural selection is not the 

 exclusive means of modification, Darwinians and non- 

 Darwinians have equally before them the problem to dis- 

 cover what these other laws are which are co-efficient in the 

 production of new species, and what part each of these 

 plays in producing the final result. Until this is accom- 

 plished we can hardly consider the great problem of the 

 Origin of Species as solved. Towards the solution of it, 

 however, the labours of Mr. Darwin will ever be held as 

 having contributed a larger share than those of any other 

 naturalist. When we look at the title-page, and see that 

 a work which has produced a greater revolution in the 

 scientific thought of the day than any published in this 

 country since Newton's " Principia " is yet only in its 

 eleventh thousand, and reflect that, although this is not a 

 small sale for a scientific work, yet books which contain 

 the germ of no new thought, and contribute not one iota 

 to our sum of knowledge, have sold their hundreds of 

 thousands, we cannot but think that in the coming age, 

 when the people will really care about science, our de- 

 scendants will regard this unworthy fact in the light that 

 we do the unpopularity of the writings of Milton and 

 Goldsmith during their lifetime.- 



We must not omit to mention a very useful addition, for 

 the unscientific reader, made to this edition, in the shape 

 of a glossary of the principal scientific terms used, pre- 

 pared by Mr. W. S. Dallas. 



Alfred W. Bennett 



MAXWELL ON HEAT 



Theory oj Heat. By J. Clerk Maxwell, M.A., LL.D, 

 (London : Longmans and Co. 1872.) 



IT is very seldom that we meet with a book so instruc- 

 tive and delightful as Prof. Maxwell's " Theory of 

 Heat." It has peculiar claims upon the student of Physics, 

 inasmuch as it supplies a want which has been long and 

 widely felt. The point of view is undoubtedly a new one, 

 and to enable our readers to perceive the value of the 

 book, we ought to make a few remarks upon the kinds of 

 text-books that we have hitherto had. In these books the 

 aim has been to inform the student's mind, and the 

 fault to inform it too minutely and too exclusively. 

 They have beea of two classes — elementary books, in 

 which the information is given in a popular manner, 

 and advanced books, through the pages of which mathe- 

 matical formate are very liberally interspersed. 



In reading such a book the strength of the student's 

 mind is devoted to one or at most two objects. If the 

 book be elementary, he is bent upon acquiring a good 

 knowledge of the facts, along with a knowledge, more or 

 less complete, of the experimental methods by which these 

 facts have been obtained. If, on the other hand, the book be 

 an advanced one, his strength is devoted to grappling with 

 and overcoming its analytical difiiculties. But after he has 

 studied both classes of text-books, he rises from their pe- 

 rusal with the belief that there is something wanting before 

 he can have a thorough grasp of the subject, and a clear 

 view of its truth and beauty. He has followed the experi- 

 menter only too zealously into his elaborate and accurate 

 calculations, or it may be the mathematician into his pro- 

 found investigations, and he now begins to reahse the 

 truth of the poet's saying- 

 He who hath watched, not shared, the strife 

 Knows how the day hath gone, 



and to sigh for some elevated spot from which he may 

 obtain a clear view of the whole field. He hears vague 

 rumours that the caloric battalions and their allies the 

 corpuscular forces, have lost the day, but he wishes to see 

 their discomfiture more completely with his own eyes. 



Such a point of view is afforded by Prof. Maxwell. 

 He has — wisely, we think — confined himself to this one 

 object, to give the student a clear logical view of the 

 whole subject ; nor has he broken the unity of his treat- 

 ment by going into details, whether experimental or 

 mathematical. Every true student of physics should read 

 this book, and he will unquestionably find it a most delight- 

 ful study. He will, we venture to say, rise from its perusal 

 with a much truer and wider conception of the science of 

 heat ; and if he then wants more detailed information 

 upon any branch, he may consult one of the ordinary 

 text-books. Another beauty of the book is the accuracy 

 and completeness of its historical notes. The author 

 has successfully combined the part of historian and 

 that of logician, and has given us very many valuable 

 references to original memoirs, in which we may see for 



