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Show 292 DARWINISM CTIAI'. springs from a very small area on the side. of the brc~st. Mr .. Frank E. Beddard, who has kindly cxa.nuncd a spec1mcn f01 me, says that "this area lies upon the pectoral muscles, and near to the point where the fibres of the muscle convc~·gc towards their attachment to the humcrns. The plumes ansc, therefore, close to the most powerful muscle of the body, and ncar to where the activities of that muscle would be at a. maximum. Furthermore, the area of attachment of the plumes is just above the point where the arteries. ~nbd n~rves f01~ tho supply of the pectoral muscles, and nmg 1 ounng rcg10ns,. leave the interior of the body. The area of attachment of the plume is, also, as you say in your letter, just :tbovc th ' junction of the coracoid and sternum." Orna~cntal plumes of considerable size rise from the same part m many oLher species of paradise birds, sometimes extendin~ laterally in fr~n t, so as to form breast shields. They also occur m many hnmmlllg birds and in some sun-birds and honey-suckers; and in all these cases' there is a wonderful amount of activity and rapid movement indicatincr a surplus of vitality, which is able to manifest itself ' in tho devb elopment of these accessory p l umcs. 1 In a quite distinct set of b~r~s, the gallinac~re, we find t.l~ o ornamental plumage usually ansmg from .very d1ffcrent parts, 1~1 the form of elongated tail-feathers or t~1l-covcrts, and of .runs or hackles from the neck. Here the wmgs arc comparn,tJ vcly little used the most constant activities depending on tho legs, since the gallinacere are pre-emin~ntly walk~ng, rmmitw, <Uld scratching birds. Now the magmficent tram of . tho po:teo<'k -the crrandest development of accessory plumes m th1s or<l<'r -sprinbg s from an oval or circular area, ah~ut t h roc m. e1 1 cs . 111 diameter just above the base of tho tml, and, therefore, situated 'over the lower part of the spinal column ncar the in crtion of the powerful muscles which move the hincl limll~ and elevate the tail. The very frequent presence of ncck-rnfls or breast-shields in the males of birds with accessory plmnes may be partly due to selection, because they must serve as n, protection in their mutual combats, just as docs the lion's or t.ho horse's mane. The enormously lengthened plumes of the lnnl of paradise and of the peacock can, however, h:we no such usc, 1 For activity and pugnacity of humming-birds, see 'l'ropical Natu re, PP· 130, 213. x COLOUR::; AND ORNAMENTS CHARACTERISTIC OF SEX 293 but must he rather injurious than beneficial in the bircl's orclinary life. . The fact th:~t they have been developed to so grcn,t an extent m a few species is an indicn,tion of such perfect n,daptn,tion to the conditions of existence, such complete success in the hn,ttlc for life, that th rc is, in the adult male at n,ll events, a mplus of strength, vitnJity, n,nd growth-power which is able to expend itself in this way without injury. Thn,t such i. the case is shown by the great abundance of most of the species which possess these womlcrful superfluities of plumage. Birds of pamdisc arc among the commonest birds in Now Guinea, and their loud voices can be often heard when the birds themselves arc invisible in the depths of the forest; while lnclin,n Rportsmen have de. cribed the pc:tfowl as being so a,bundant, that from twelve to fifteen hundred have been seen within an hour at one spot; and they range over the whole country from the Himabyas to Ceylon. Why, in allied species, the development of accessory plumes hn,s taken different forms, we are unable to say, except that it mn,y be due to that individual varia,bility which hn,s served as the starting-point for so much of what seems to us strange in form, or fantastic in colour, both in the animal and vegetable world. Development of Accessot·y Plumes and their Display. If we have found a verct causa for tho origin of ornamental appendages of birds and other animals in a surplus of vital energy, leading to abnormal growths in those parts of tho integument where muscular and nervous action arc greatest, the continuous development of these appendages will result from the ordinary action of natural selection in preserving the most healthy and vigorous individuals, and the still further selective agency of sexual struggle in giving to tho very strongest n,nd most energetic the parentage of the next gcncratiqn. And, as all the evidence goes to show that, so far as female birds exercise any choice, it is of "the most vigorous, defiant, and mettlesome male," this form of sexual selection will act in the same direction, and help to carry on the process of plume development to its culminn,tion. That culmination will be reached when the excessive length or n,lmndancc of the plumes begins to be injurious to the hea,rcr of them· and it may be this check to the further lengthening of the p~a,cock't |
