OCR Text |
Show 242 DARWINISM OIIAl'. were fa,r less common, so that Mr. Bates estimated the proportion in some cases as not one to a thousand. ~c~orc giving an account of the numerous remarkable case~ of m1m1cry in other pttrts of the world, and between varwus (rroups of insects and of higher animal , it will be well to explain briefly the u c and purport of the phcnomcno11, and also the mode by which it has been brought about. Ifow llfimicry has been Prod1tced. The fact has been now established that the Hcliconid:P posse. s an offensive odour and taste, which lead to thci1 being a.lmost entirely free from attack by insectivorous crcatnrc ·; they po scss a peculiar form and mode of fl ight, and do not seek concealment; while their colour - aJthongh very varied, ra,nging from Jeep hluc-bhck, with white, yellow, or vivid red lmnds ~wd . pots, to the mo. t delicate scmit ratt ~ parent wings adornctl with pale brown or yellow marki ngs arc yet always very di. tinctivc, and nnlikc tho. c of all tltc other families of butterflies in the sa,mc country. 1t is, therefore, clear that if any other buttcdiic.· in the r-;amc region, which are eatable and ufJ'cr rrreat persecution from insectivorous anim<tls, should come to rc mblc any of these uneatable specie. so clo ely as to be mist;tlcen for thPm hy their enemies, they will obtain thereby immunity from p1·rsccution. Thi i. the obvious and sufficient reason whr the imitation is useful, and therefore why it occurs in natnrc. ·we have now to explain how it has probably been brought alHn tt, and also why a still larger number of persecuted gronp:-> h:t vc not availed themselves of this simple means of protection. From tho groat abundance of tho Holiconid re I all m cr tropical America, the vast number of their genom aml species, and their marked eli tinctions from all other bnttcrtii!•s, it follow that they constitute a gronp of high antiqni ty, which in the course of ages has become more and more .·pcri,tliscd, and owing to it. peculiar ad vantages has now hccomc a Jominant and aggres ive race. But when they first arose from some ::mcestral species or gronp which, O"\ving to the food 1 These butterflies nrc now divided into two sub-famili es, one of which is placed with il1 e Danaiclro; hut to avoi<l confu~ion I shall always speak of the American genera under the ol<l term Heliconiclro. WARNING COLORATION AND MIMICRY 243 of the larvre or some other cause, possessed disagreen.hle juices that caused them to be dislike~ ~y the usual e~1emies of their kind, they were in all probability not very different either in form or coloration from many other butterflies. They would at that time be subject to repeated attacks by insecteaters and, even if finally rejected, would often receive a fatal i~1jnry. Hence arose the necessity for so~e d~stinguishing mark, by which the cl~vourers of bt~ttcrfl1es m general miaht learn that these particular buttorflws were uneatable; and every variation leading to such distinction, whether by form, colour, or modo of flight, was preserved and accumulated by natural selection, till the anccstra.l Heliconoicls became well distingui. ·heel from eatable butterflies, and thenceforth comparatively free from 1~orsccution. . Then they had a good time of it. They acqmred lazy habits, and flew about slowly. They increased abundantly and spread all over. ~he c~untry, their larvre feedino· on many plants and acqmrmg cbfferent habits · while the 0 butterflies themselves varied greatly, and colour 'bein(l' useful rather than injurious to them, gradually diverged into the many coloured and beautifully varied forms we now behold. But, during the early stages of this process, some ~f the Pieridre inhabitina the same district, happened to be sufficiently like so~o of the Helieonidre to be occasionally mistaken for them. These, of course, survived while their companions were devoured. Those among their descendants that were s.til~ m?rc like Heliconidre again survived, and at length the ImitatJOn would become tolerably perfect. Thereafter, as the protected group diverged into distinct speci~s of many different colour .. , tho imitative group would occasiOnally . be a_ble to follow It with similar variations,-a process that 1s gomg on now, for Mr. Bates informs us that in each fresh district he visited he found closely allied roprosc.ntative species. or varieties ~f Heliconidre and alona "\'11th them speCies of Leptahs (Pieridre), ~hich had va~ie~ in the same ~a;y s~ as still to be exact imitations. But th1s process of ImitatiOn would be subject to check by the increasing acutene.ss of birds and other animals which, whenever the eatable Leptahs became numerous, would surely find them out, and wo~ld .the~ probably attack .hoth these and their friends the Hehcomdre m order to devour |