536 



NA rURE 



[October 6, 1892 



The author is especially candid and straightforward in 

 bringing forward the evidence in favour of an explana- 

 tion he is about to attack. After thus fairly showing 

 the strength of the opposed position, he proceeds to 

 reject it for reasons which will strike the instructed and 

 uninstructed reader alike as singularly inadequate. 

 Examples of this method occur continually throughout the 

 volume. As an example may be selected his treatment 

 of the opinion that the light of phosphorescent organs 

 enables certain deep-sea animals to see. He admits the 

 existence of eyes, the prevalence of phosphorescence, the 

 intensity of the light emitted, the existence of " lens-like 

 transparent bodies serving to concentrate the rays of light," 

 the fitness of the light to illuminate the prevalent colours. 

 In spite of all these facts the author believes that all deep- 

 sea colours are unseen and meaningless for the following 

 remarkable reason :— " The presence of well-developed 

 eyes, or the total absence of these structures, are, as has 

 been explained, intelligible on the theory of abyssal 

 light ; not so the existence of eyes in an intermediate 

 condition. The inevitable conclusion, therefore, from 

 these facts appears to be that the brilliant and varied 

 coloration of deep sea animals is totally devoid of 

 meaning ; they cannot be of advantage for protective pur- 

 poses, or as warning colours, for the simple and sufficient 

 reason that they are not seen " (p. 37). The author car- 

 ries this conclusion to its logical end, and, pointing to the 

 resemblance of deep-sea forms to their shallow-water 

 allies, and the existence of protective resemblances in 

 both, he maintains that " if natural selection has been 

 the cause in the one case, it ought to be in the other. 

 .... The question therefore is pressing : need natural 

 selection be responsible for the coloration of the shallow- 

 water forms ? " (p. 38). A somewhat large conclusion to 

 base on the fact that the eyes of certain deep-sea animals 

 are in process of degeneration ! The author admits that 

 the absefice of eyes is no argument for his views ; and yet, 

 in every such instance, a gradual process of degeneration 

 has been passed through. He gives us no reasons for re- 

 jecting the opinion that the casesupon which he bases such 

 startling conclusions are merely tending in the same direc- 

 tion ; indeed, elsewhere (p. 11) he insists on the probability 

 that such biological changes are still progressing. It 

 is indeed most probable that light is far from widespread 

 or intense on the floor of the ocean, and that, therefore, 

 eyes to be of use must be unusually efficient, while, unless 

 absolutely necessary, they are likely to disappear. We 

 meet, in fact, with a case somewhat parallel to that of 

 beetles on oceanic islands in tempestuous zones, where 

 selection operates in opposite directions— towards un- 

 usual powers of flight, when flight is a necessity, and 

 towards the total loss of any such capacity when it is un- 

 necessary. Thus, among deep-sea fish we find eyes of 

 immense relative size, as well as those which are degene- 

 rate. And the phosphorescent organs of certain fish 

 {Ceratias) appear to emit a light which is invisible to the 

 degenerate eyes of the possessor, but serves to attract 

 other and better endowed fish upon which the Ceratias 

 feeds. The frequency of this degeneration among the 

 deep-sea Crustacea, which impressed the writer so pro- 

 foundly, may very probably be due to conditions of life 

 which render vision less necessary for them than for many 

 NO. I 197, VOL. 46] 



other groups, and this is especially probable since many 

 shallow-water genera are sightless, as is abundantly 

 shown in the book itself (p. 36). 



On pages 115, 116, the author adopts Prof. Weldon's 

 objection to the usually received interpretation of the 

 whiteness of certain eggs, and the under-sides of fish, 

 porpoises, &c., which are seen from below, on the ground 

 that snow-flakes appear almost black when seen from be- 

 neath against the bright sky. The original suggestion is 

 due to neither Mr. Wallace nor to the present writer, but 

 to Erasmus Darwin, writing very nearly one hundred 

 years ago. The objection entirely misunderstands the 

 hypothesis, at any rate so far as the eggs are concerned. 

 If an egg, lay exactly over one of the interstices in the 

 nest, it would, of course, shut out the sky altogether, 

 and when viewed from some distance through the opening 

 would appear dark like the nest itself. There would be 

 no question of its appearing against a back-ground of sky. 

 As a matter of fact, no such continuous back-ground can 

 be seen through the nest at all. Minute bright points 

 are seen through the interstices of the nest, and those of 

 the leaves and branches above and below it. The hypo- 

 thesis in question suggests that part of the bright white 

 side of an t.g'g, viewed obliquely from below through an 

 interstice, may be mistaken for one of these bright points. 

 The hypothesis may be erroneous, but it is not to be set 

 on one side by a criticism which fails to understand it. 

 In the case of the fish, the question is complicated by the 

 absorption of light by the layer of water. 



The reader who finds that the above-quoted criticism 

 is held to be destructive by the author, may be excused 

 strong language when he meets with the following sen- 

 tence only seven pages further on:— "Among pelagic 

 fish it is common to find the upper surface dark-coloured 

 and the lower surface white, so that the animal is incon- 

 spicuous when seen either from above or below." 



The chapter on Warning Coloration is one of the most 

 valuable parts of the book, for in it we meet with a solid 

 contribution to the subject in the form of some interesting 

 experiments conducted by the author upon the animals in 

 the Zoological Gardens. Many of the results are of ex- 

 treme interest, and are a further proof of the difficulty of 

 the investigation, and the great care with which it must 

 be conducted if the conclusions are to be depended upon. 

 It has been already suggested that some of the results may 

 be perhaps explained by the fact that the insect-eating ani- 

 mals chosen for experiment are restricted to a very mono- 

 tonous or very scarce insect diet. In some rather extensive 

 experiments made by the present writer upon a mar- 

 moset, it appeared that the animal possesses a most keen 

 appreciation of the meaning of warning characters, but 

 the individual in question was accustomed to be fed on a 

 very varied diet. The discussion of the details given in 

 this chapter cannot now be attempted, but it may be 

 safely affirmed that there is nothing which is fatal to the 

 theory of warning colours, when we admit, as we are of 

 course bound to do, that even unpalatable animals have 

 their special enemies, and that the enemies of palatable 

 animals are not indefinitely numerous. 



Further criticism of the arguments is rendered impos- 

 sible on the present occasion by the exigencies of space. 

 Certain obvious misstatements call for correction, such as 



