200 



NA TURE 



[December 27, 1900 



and excretory activity should take the form of colour at 

 all. It can be admitted in a general way that increased 

 nervous stimulus might lead to increased secretion and 

 excretion generally. But there is no evidence of any 

 kind brought forward in support of this — the evidence 

 which Mr. Cunningham asks us to accept in favour of 

 his view centres round local deposits either of osseous 

 or horny matter, &c., in the case of mammals, or of 

 pigment. Colour, as such, is meaningless from the 

 •Lamarckian standpoint, and we have a right to ask 

 where is the "use" (in the Lamarckian sense) which 

 accounts for the accumulation of such deposits in definite 

 stripes and patterns in one part of a wing or feather. If 

 it be suggested that the definite arrangement is the 

 external expression of an internal law of growth, then 

 the diversity of pattern among allied species remains 

 unexplained ; and, if we allow the direct action of light as 

 a cause of pattern, we are invoking an improbable and 

 an unproved principle. It must be presumed that the 

 author considers his experiments on the coloration of 

 "the Pleuronectidae, to which he refers on p. 41, as a 

 sufficient proof of " the direct effects of stimulations." 

 At any rate this is the only instance specifically men- 

 tioned which has any direct bearing on the question of 

 the action of light as a colour-producing stimulus. It 

 appears to the writer, however, that the results which 

 Mr. Cunningham made known in 1896 are open to 

 another interpretation from the point of view of the 

 selection theory, but the discussion of this point would 

 be out of place in the present notice. 



If the "nervous excitement" theory of colour is con- 

 sidered sufficient as regards deposition of pigment, the 

 production of colour by purely physical methods, such as 

 striation and lamination, has yet to be faced by the 

 Lamarckian school. This aspect of sexual coloration 

 is very lightly dealt with by Mr. Cunningham, although, 

 in the case more especially of birds and insects, some of 

 the most gorgeous male characters are due to such 

 colours. It is, perhaps, hardly right to attribute unfair- 

 ness to an author who evidently has done his best 

 to give due consideration to the doctrine which he is 

 opposing, but impartial readers of the present book who 

 happen to be acquainted with the subject at first hand 

 cannot fail to detect the anti-selectionist bias which here 

 and there makes itself manifest in Mr. Cunningham's 

 pages. Nowhere is this more obvious than in the fifth 

 chapter, dealing with the secondary sexual characters of 

 insects. The bias is no doubt unconscious, but when 

 we consider that among butterflies, to which the author 

 devotes the larger part of the chapter, this kind of 

 'colouring plays such a very important part, the treat- 

 ment will be found, to say the least, disappointing by 

 those who wish really to know what the Lamarckians 

 have to say on this subject. The explanation offered is 

 that the colours — pigmentary and physical not being 

 separated — are caused by the direct action of light : 



" If we assume that variations in definite directions are 

 excited by external conditions, in this case principally by 

 light of different colours, the facts become intelligible in 

 •general," &c. (p. 243). 



• "By exposure I mean the kind of light to which the 

 surface is exposed, and I believe that in Lepidoptera the 

 coloration has chiefly been determined by the quality of 

 the light" (p. 245). 



NO. 1626, VOL. 63] 



We do not believe that the author will make many 

 converts to his amended Lamarckism by such explana- 

 tions as these. When and how does the light act, and 

 how can such action produce pattern ? Is the pattern 

 focussed on to the wing by some mysterious agency 

 capable of acting as a lens, or how does the light of 

 different " qualities " (? wave-length) get separated so that 

 each component produces its proper chromatic effect ? 

 Upon what organic tissue can light produce such direct 

 photo-chromatic action, and does such action take effect 

 through the skin of the pupa, in which stage the colours, 

 both pigmentary and physical, are formed in the rudi- 

 mentary wing ? It seems to the writer that, instead of 

 the facts being made " intelligible in general " by invok- 

 ing such strained explanations, they have been rendered 

 hopelessly inexplicable from the Lamarckian standpoint 

 For example, to take his own illustration, does Mr. 

 Cunningham wish us to believe that the hind-wings of 

 Catocala nupta have been exposed to red and black light 

 for generations — if so, where is the evidence ? Does he 

 seriously believe that the mottled-grey of the fore-wings, 

 which, as he admits, makes them " indistinguishable on 

 surfaces of bark or rock" (p. 246), is better explained by 

 the direct action of the light from such surfaces than by a 

 straightforward admission of natural selection for the pur- 

 poses of protection ? If he does prefer this explanation 

 it must be left for the readers of this review to consider 

 whether any bias is displayed by the author of the work 

 under notice. ^ J 



In discussing the colours of Lepidoptera, the author 

 begins with those cases in which there is mimicry com- 

 bined with sexual dimorphism, the females being, gener- 

 ally speaking, the mimetic forms. His object in doing 

 this is to bring into prominence the undisputed fact that 

 in such cases the two sexes have different habits. His 

 contention, of course, is that the habits have produced 

 the differences between the sexes. But if we do not 

 admit this^and selectionists will not accept this doctrine 

 until some much stronger evidence is forthcoming — the 

 whole force of the argument is lost, because the difference 

 in habit is also precisely in harmony with the require- 

 ments of the selection theory. What is really wanted to 

 make this argument of any value is the proof that in 

 species which are not mimetic, but which are nevertheless 

 sexually dimorphic, there is a sufficiently marked differ- 

 ence between the habits of the sexes to account for the 

 greater brilliancy of the one sex. So far as the writer 

 has observed, there is no marked difference of habit lead- 

 ing to different exposure, for example, between the male 

 and female in species of Colias, Anthocharis , Pieris^ 

 Gonepteryx, Hipparchia, Satyrus, Argynnis Hesperia^ 

 &c. Yet in all these there is in most of the species a 

 marked dissimilarity of sex. To narrow down the issue,^ 

 why, on Lamarckian principles, should the male orange- 

 tip {A. cardamines) have an orange patch on the fore 

 wings, and the female Colias edusa a row of orange spots 

 round the black border ? Why should the , male 

 Goftepteryx rhamni be bright yellow and the female 

 greenish-white ? 



In all cases where direct observation is wanting, or 

 where there is no observable difference of habit be- 

 tween the sexes, the author assumes that there must be 

 some such difference. Of course this is a very easy way 



