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Show 252 DARWINISM CHAP. difference, in the important character of the neuration of tho wino- , between the e butterflies, which really belong to very dist~1ct and not at all closely allied genera. Other important characters are-(1) The existence of a small basal cell in thr hind wings of Ituna which is wanting in Thyridia; (2) the division of the cell between the veins 1b and 2 of tlw hind wings in the former genus, while it is undivided in t lw latter; and (3) the existence in Thyridia of scent-producing tufts of hair on the upper edge of the hind wing, while in Ituna these arc wanting; but in place of them arc cxtcnsihlc processes at the end of the abdomen, nl o emitting a po~werfnl scent. These differences characterise two marked subdivi. ious of the Dana.oid Heliconinre, ea ...:h containing several distinct genera; and the. e subdivision are further eli tinguishcd by YCl'J different forms of larvre, that ~o which Ituna belongs h<LYing from two to four long threadlike tentacles on the hack, ·while in that containing Thyridia these are always absent. The former usually feed on Asclepiadere, the latter on Sola.nace;c or 'croph ulariaccre. The two species figured, though belonging to such distinet and even remote genera, have acquired almost identi cal tints and markings so as to be deceptively alike. The surface of the wings is, in both, transparent yellowish, with black trans- • verse bands and white marginal spots, while both have simihn· black- and white-marked bodies and long yellow antenna:. Dr. Muller states that they both show a preference for the same flowers growing on the edges of the forest paths.l 'Ve will now proceed to give the explanation of these curious similarities, which have remained a complete pnzzlc for twenty years. Mr. Bates, when first describing tlwm, sugge ted that they might be due to some form of parallel variation dependent on climatic influences; and I my:-;elf adduced other cases of coincident local modifications of colour, which did not appear to be explicable by any form of mimicry. 2 But we neither of us hit upon the simple explanation given by Dr. Fritz Muller in 1879. His theory is founded on the assumed, but prolJahlc, 1 From Professor Meldola's translation of Dr. F. Muller' paper, in Proc. Ent. Soc. Lond., 1879, p. xx. 2 Island Life, p. 255. JX WARNING COLORATION AND MIMICRY 253 fact, that insect-eating birds only learn hy experience ~o distinguish the edible from the incclihle butterflies, and m doing so necessarily sacrifice a. certain number of the l:ttte~·· The quantity of insectivorous birds in tropical Amenca IS enormous; and the number of young birds which every. year have to learn wisdom by experience, as regards the species of butterflies to be caught or to be avoided, is so great that the sacrifice of life of the inedible species must be, consiclcrah1c, and, to a comparatively weak or scarce species, of vita] importance. The numl>cr thus sacrificed will be fixed l~y tho quantity of young birds, and ?Y the ~um.her of e.xpencnces requisite to cause them to avmd the mc~hb1.c .spccw. for ~he future, and not at all by the numbers of mchvidu:Lls of wh10h each species consists. Hence, if two species arc o much alike as to be mistaken for one another, the fixed number annually sacrificed by inexperienced bird will be divided between them a.nd both will benefit. But if the two species are very unequ~tl in numbers, the benefit will be comparatively slight for the more abundant species, but very. w·ca.t for the rare one. To the latter it may make all the d1ftcrence between safety and destruction. To give a rough numerical example. Lc.t ns supp?sc ~hat in a given limited district there arc two spec10 of ~cl.Ic?md< , one consistina of only 1000, the other of 100,000 mclivHlun.L, and that the bquota required annually in the same district for the instruction of young insectivorous birds is 500. By tl:e larger species this loss will be hardly fe~t; to the ~mal~cr It will mean the most dreadful persecutiOn resultmg m a loss of half the total population. But, let the two species become superficially alike, so that the birds see no difference between them. The quota of 500 will now be taken from a combined population of 101,000 butterflies, and if prop~rtionate numbers of each suffer, then the weak speCies ,nn only lose five individuals instead of 5.00 as it .did. before. Now we know that the different species of Hehcomdre are not equally abundant, some being quite rare ; so tha.t ~he benefit to be derived in these latter cases would be very nnportant. A slight inferiority in rapidity of flight or in pow~rs of eluding attack mio·bt also be a cause of danger to :m medible species of scan~y numbers, and 1n this ca c too the being |