CHAPTER VII. THE TWO BEST ILLUSTRATIONS. "More particulars Must justify my knowledge." -Cymbeline. Act ii., sc. 4. WHEN a particular department of nature is selected for the exposition of some general principle, the object may be to illustrate the action of what is believed to be a universally acknowledged law of nature, or else to test the correctness of a tentative theory. There can be no doubt whatever as to the advantage, so far as explanation is concerned, of a concrete illustration of an abstract principle. "The philosopher," says Dugald Stewart, "whose mind has been familiarised by education and by his own reflections to the correct use of more comprehensive terms, is enabled. . . to arrive at general theorems; which, when illustrated to the lower classes of men in their particular applications, seem to indicate a fertility of invention little short of supernatural."* This principle is duly recognised by Mr. Romanes, when he says that "perhaps the proof of Natural Selection as an agency of the first importance in the transmutation of species may be best brought home to us by considering a few of its applications in detail."+ Now it is perfectly legitimate for those who approach the consideration of a particular group of phenomena, to * Elements of The Philosophy of the Human Mind. Part i., chapter iv. + Darwin and After Darwin. p. 316. assume the truth of the given law which they believe to have been already proved. Thus Mr. H. F. Osborn says that "the law of Natural Selection is well established, and no more under discussion." Those who believe this may confidently enter on a new field of enquiry, expecting to find illustrations of the action of Natural Selection there as elsewhere. That is the spirit in which Mr. Poulton entered on his most valuable investigations. He says: "Any scientific work which I have had the opportunity of doing has been inspired by one firm purpose-the desire to support, in however small a degree, and to illustrate by new examples those great principles which we owe to the life and writings of Charles Darwin, and especially the pre-eminent principle of Natural Selection." (Preface. p. xiii.) That is the spirit which inspired Mr. Darwin when he penned the following passage : "When both sexes of birds are so obscurely coloured that it would be rash to assume the agency of sexual selection, and when no direct evidence can be advanced showing that such colours serve as a protection, it is best to own complete ignorance of the cause, or, which comes nearly to the same thing, to attribute the result to the direct action of the conditions of life."-(Descent of Man. p. 491.) What is this but a predetermination to know nothing but Selection, Natural or Sexual? Apart from one of these explanations, we can know nothing! In the same way, Mr. Romanes takes for granted the action of Natural Selection in his attempt to solve the problem presented by instinct. Moreover, it was necessary, as a mere matter of historical sequence, that the proof of the transmutation of species as against the fixity of species, of evolution as against sudden creation, of transmutation of species and organic evolution by means of Natural Selection rather than by other means, should have been first established. Mr. Henry Walter Bates promulgated the theory of mimicry, having been first convinced of the truth of Natural Selection; and no one would have thought of instinct being developed by Natural Selection who had not first been convinced that it was an all-embracing law of nature. Such a treatment of the subject is perfectly legitimate for those who are certain that Natural Selection is a great law of nature. Still it may be very much questioned whether such a conviction tends to produce a perfectly unprejudiced treatment of the subject. But the enquirer into the truth of Natural Selection regards a new sphere of nature as simply offering a new test of the theory. He can take nothing for granted but what can be proved; he will try to enter on the new enquiry with an open mind and with a perfect absence of all bias. This attitude of mind is well expressed in the following statement, made by Professor Eimer, in concluding his work on Organic Evolution: "Our duty is work; our right is free investigation; our satisfaction the establishment of a grain of truth for the benefit of mankind; our hope, knowledge.”—(p. 435.) Animated, if possible, by this spirit, we have now to consider how far it is true to say that defensive colouring and instinct afford either the best illustrations of the supposed law of nature, or the best proofs of that wellknown theory. (a) DEFENSIVE COLOURING. "You shall play it in a mask." -Midsummer Night's Dream. Act i., sc. 2. Mr. Romanes selects defensive colouring as affording the best illustration of the action of Natural Selection. "Among all the possible fields from which evidences of this kind (the application of the theory in detail) may be drawn, the best is that which may be generically termed defensive colouring."(Darwin and After Darwin. p. 316.) Mr. Henry Walter Bates was the first to apply the theory of Natural Selection to the phenomena of animal colouring. In his classical paper entitled "Contributions to an Insect Fauna of the Amazon Valley, Lepidoptera: Heliconidae," he described the manner in which one species of butterfly, protected by a nauseous taste, was imitated or mimicked by another insect not so protected, and he attributed this strange resemblance to the action of Natural Selection. This interpretation was afterwards extended to those species which mimic the colour and form of some of the objects by which they are ordinarily surrounded. The existence of certain very conspicuously coloured insects presented an unexpected difficulty; but Mr. Wallace met this objection with an explanation which has been generally accepted as a remarkable confirmation of the theory. Such is the historical order of the application of Natural Selection to the problem of defensive colours. But we shall perhaps understand the question better if we realise the logical explanation which binds together these different phenomena; for, as Mr. Poulton says: "in this, as in so many other cases, the steps by which the subject is best approached are almost exactly opposite to the historical steps by which it was gradually understood."† The logical nexus, which binds together all the phenomena of defensive colouring, may be stated thus. At the outset, the advocate of Natural Selection, already convinced, as we have seen, that his theory is a great law of nature, has no hesitation in assuming that defensive colouring is an illustration of the action of this great principle. * The Transactions of the Linnean Society. vol. xxiii., part the third, pp. 495-566. The Colour of Animals. p. 220. But if this is so, a difficulty at once occurs in connection with those animals which are conspicuously coloured, and yet enjoy immunity from attack. This difficulty is re moved by the discovery that these creatures have certain disabilities attached to them are hard to catch, difficult to swallow, or not good to eat; and that it is therefore of advantage, both to themselves and their enemies, that they should have some distinct "advertisement of inedibility." This advantage being once admitted, it is obviously beneficial to a species which is not thus protected and which is good to eat, that it should mimic those which are difficult to catch or unpleasant to eat. I venture to affirm that we cannot find in connection with any scientific investigation a more splendid illustration of the extension of a theory so as to afford an all-embracing explanation of varied phenomena. It only remains for us to ask whether the actual facts agree with this great and striking generalisation. It is interesting to observe that the facts were known. long before the time of Mr. Bates. Boisduval, in 1836, drew attention to the resemblance between butterflies of different genera and species;* and in 1815 Kirby and Spence speak of insects "imitating the colour of the plants, or parts of them, which they inhabit"; and refer to "the spectre tribe (Phasma) as going still further in this mimicry." Therefore Mr. Bates cannot be credited with the invention of the term "mimicry," except in so far as it is understood to mean that mimicry is produced by the action of Natural Selection. Before dealing with the different classes of phenomena. embraced under the generic term of defensive colouring, it may be well to consider certain features of this method of K # Species Général des Lepidoptères. pp. 372-373. + Introduction to Entomology. p. 405. |