234 



KNOWLEDGE. 



[October 1, 1805. 



quantity of water in suspension. It is too often overlooked 

 that optical figures in the atmosphere are not really plane 

 but solid figures, geometrically. 



I am not aware that any satisfactory explanation of 

 involved rainbows has been published. My own theory, 

 suggested, however, with all humility, is that they show the 

 depth of the rain bank, and are a perspective foreshortening 

 of rings from the base of an (imaginary) cone of particles, 

 and only limited in number by the limits of the refraction 

 angles. 



A drop of rain with crystalline nucleixs, i.e., nascent hail, 

 will give rise to curious optical phenomena. But I do not 

 refer these figures to the varying size of rain-drops, a theory 

 which badly agrees with the observed proportionate reces- 

 sion of the rings. — Samuel Baeisek. 



Elmsett, Ipswich. 



COLOUES OP BUTTERFIIKS. 



To the Editor of Knowledge. 



Sir, — It is difficult to reply to Mr. Miller on such com- 

 plex problems with the limited space at my disposal. I 

 will, however, endeavour to state my answers to his 

 queries as shortly as possible. 



(1) Mr. Miller says he cannot see the truth of the truism, 

 that some insects do not require protection, while others 

 do require it. The answer is that all insects, in order to 

 survive, require some form of protection unless their 

 power of rapid flight is superior to that of their enemies. 

 This protection may take the form of protective resem- 

 blances, mimicry, or warning colouring. 



(2) Mr. Miller denies that natural selection is an 

 " environmental force." This is merely a question of the 

 meaning of words. Natural selection is, so far as it is 

 accepted, a force or power whereby those individuals, 

 which are most suited to their enNironment, survive in the 

 struggle for existence ; those less in conformance with 

 their environment "going to the wall." This is a fact, 

 whatever name we give to the acting power ; the only 

 doubtful point is, whether natural selection will explain 

 everything. 



(3) Mr. Miller objects to the " metaphorical " use of 

 the word "mimicry." This I explained in my last 

 letter. Mimicry implies conscious imitation, whereas in 

 the cases we are considermg there is supposed, by Darwin 

 and Wallace, to be no conscious effort, as Lamarck 

 formerly stated. 



(4) Mr. Miller queries how can hereditary transmission 

 increase resemblance {i.e. — protective resemblance and 

 mimicry) ; and how can accidental resemblances be trans- 

 missible ? Hereditary transmission is bound to increase 

 resemblance in such cases if the species is to survive at all, 

 simply because all those of lesser resemblance are killed, 

 and only the most modified escape the attacks of enemies. 

 In each generation some individuals will have more pro- 

 tective resemblance than others ; because no two animals 

 are alike, and these only will survive, and thus the 

 resemblance is increased in successive generations. 



(5) Mr. Miller again asks, " what has this to do with 

 the origin of species '.' " I can only say that Darwin and 

 Wallace's explanation of the origin of species was given in 

 their theory of natural selection, which is briefly as 

 follows : In any animal or plant there is a rapid increase 

 of numbers by reproduction ; but the total numbers are 

 stationary — hence the struggle for existence. As no two 

 individuals are alike, there is variation, and this is trans- 

 mitted by heredity to successive generations. Hence 



follows the survival of the fittest, leading to the survival 

 only of those best adapted to their environment. By 

 means of protective colouring and mimicry, many species 

 of butterflies survive and form species, which would other- 

 wise disappear. This is what the question has to do with 

 regard to the origin of species. 



(G) Mr. Miller's last query I do not quite follow. He 

 seems to imply that butterflies are most exposed to danger 

 while on the wing, and asks why are they not protected by 

 the colours on the upper surfaces of the wings '? This 

 really requires no answer. While on the wing, butterflies 

 must trust to their powers of flight and take their chance ; 

 the most obvious need for protection is while they are 

 perching with the wings folded up, especially when the 

 female is laying eggs. Hence the under surface of the 

 wings is protectively coloured. The upper surface is 

 usually bright by sexual colouring to attract the opposite 

 sex. 



C. F. Makshall, M.D. 



To the Editor of Knowledge. 



Sir, — I am sorry to see that my letter has made this 

 subject no clearer for Mr. Miller. 



In the first place, respecting the necessity of protective 

 colouring, I understand a necessity to be that which is 

 indispensable, and thus fail to see how Mr. Miller can at 

 one and the same time logically maintain that protective 

 colouring is always necessary and yet that many insects 

 exist without it. To me one statement appears to 

 contradict the other. 



Then, as to the difl'erence which he considers I ought 

 to make between the protected and the unprotected, I 

 would point out that as the economy of no two species is 

 exactly alike, no distinct line of demarcation can possibly 

 be drawn. Insects displaying the rapid motion of the day- 

 flying moths, or the up-anddown eccentric flight of many 

 of our butterflies, would probably be little better off were 

 they difl'erently coloured. Those flying only in the night- 

 time would in the majority of cases, where their day-time 

 habits did not clash, need no special colouring beyond some 

 sombre tint ; while again, those inhabiting districts where 

 enemies were scarce or not powerful, would have small 

 inducement offered for their improvement. 



I think it probable that protective colouring occurs more 

 frequently than Mr. Miller supposes. So far as my own 

 experience goes, I have found the cases where the colour 

 of an insect is out of all harmony with its surroundings, 

 during each period of its existence, to be the exception 

 and not the rule ; and in such instances contra-balancing 

 causes may often be found, such as the unpleasant taste of 

 the magpie moth {Ahni.ras i/rossulariata). Besides, the 

 evolutionary doctrines demand, not that each form should 

 be perfect, but only improving. 



Then Mr. Miller objects to natural selection as an 

 environmental force. I do not know whether he wishes to 

 deny the existence of natural selection altogether, or only 

 objects to my terming it " a force." In the latter case 

 what label would he prefer as a substitute '? When oil and 

 water are mixed together, one rises and the other sinks by 

 what we call the force of gravitation ; so, when good and 

 bad organic forms are turned out into the world, the strong 

 rise and the weak fall, and I can see no objection to 

 terming that which makes them rise and fall "a force." 



Mr. Miller says he is not aware that any scientific men 

 ever employ metaphor in enunciating truth, and adds, 

 " certainli/ neitlier Darwin nor Waltare so uses it." Surely 

 Mr. Miller has not been very observant, or he would not 

 have made such a statement. I think it would be hard to 



