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Show 298 DARWINISM CHAP. 3. There are indications of a progressive change of colour, perhaps in some definite order, accompanying the developu~ent of tissues or appendages. Thus spots spread and f~se mto bands, and when a lateral or centrifugal exp~nswn has occurred-as in the termination of the peacocks' tram feathers, the outer web of the secondary quills of the Argus J?heas~nt, or the broad and rounded wings of many butterfhes-mto variously shaded or coloured ocelli. The fact t~at we find gradations of colour in many of the ~o:c extensive. gronpfi, from comparatively dull or simple to bnlliant and vaned hues, is an indication of some such law of development, duo probably to progressive loc~l segregation in the tissues of identical chemical or orgamc molecules, and dependent on laws of arowth yet to be investigated. 4. The colours thus produced, and subject to much individual variation, have been modified in innumerable ways for the benefit of each species. The most general modification has been in such directions as to favour concealment when at rest in tho usual surroundings of the species, sometimes carried on by successive steps till it has resulted in tho most minute imitation of some inanimate object or exact mimicry of some other animal. In other cases bright coloms or striking contrasts have been preserved, to serve as a warning of inedibility or of dangerous powers of attack. Most frequent of all has been the specialisation of each distinct form by some tint or marking for purposes of easy recognition, especially in the cas0 of gregarious animals whose safety largely depends upon association and mutual defence. 5. As a general rule the colours of the two sexes are alike; but in the higher animals there appears a tendency to deeper or more intense colouring in the male, due probably to his greater vigour and excitability. In many groups in whi!'h this superabundant vitality is at a maximum, the dcv<·lopment of dermal appendages and brilliant colours has gone on increasing till it has resulted in a great diversity between the sexes · and in most of these cases there is evidence to Hhow that ~atural selection has caused the female to retain the primitive and more sober colours of the group for purposes of protection. COLOURS AND ORNAMENTS CITARACTERISTIC OF SEX 299 Concluding Remarks. The general princiJ?les of colour development now sketched out enable us to give some rational explanation of the won~erful . amount of brilliant colour which occurs among tropic~l ~mmals.. Looking on colour as a normal product of orgamsatwn, whiCh has either been allowed free play or has been checked and modified fo: the benefit of the spe~ies, we can sec at once that the luxunant and perennial vegetatio f the tropics, by affording much more constant means of ~o~~ ealment, has rendered brilliant colour less hurtful there than m the .temperat~ and colder regions. Again, this perennial vegetatiOn suppbcs abundance of both vegetable and insect food throu~hout the year, and thus a greater abundance and greater vanety of the forms of life arc rendered possible, than whe:e. :~curren_t seasons. ~f cold and scarcity reduce the possibilities of l~fe to a mm1mum. Geology furnishes us with a~ot~er reas~n? m the fa~t, that t_hroughout the tertiary period tropical condi~I~~s .prevailed far mto the temperate regions, so that the possibihtws of colour development were still o-reater than they are at the present t~mc. The tropics, th~efore, present to us the results of ammal development in a much larger area and under more favourable conditions than prevail_ to-day. We see in them samples of the productions of an carl.Ier and a be~ter world, from an animal point of view; and this probably gives a greater variety and a finer display of colour than would have been produced, had conditions always been what they are now. The temperate zones, on the other hand, have re~cntly. suffered the effects of a glacial period of extreme se:cnty, with the result that almost the only gay colo~rcd birds they. now possess arc summer visitors from tropiCal or sub-tropiCal lands. It is to the unbroken and al~ost ~nchcckcd course of. dev~lopment from remote geological times that has prevailed m the tropics, fa vourcd by abundant food and pere~nial shelter, that we owe such superb developme.nts ~s the fnlls and crests and jewelled shields of the hummmg-birds, the golden plumes of the birds of paradise, and the resJ?len~ent train of the peacock This la,st exhibits to us ~he ~ulmmatwn of that marvel and mystery of animal colour whiCh Is so well expressed by a poet-artist in the following |