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Show 118 DARWINISM CHAP. several seconds, a large part of the body is submerged. Now these birds still have the plumage pervious to w<ttcr, and so are liable to be drenched and sodden; but they have a.lso the faculty of giving these drenched feathers such a good. sha.king tha.t flight is practicable a. moment a.ftcr leaving the wa.ter. Certainly the water- thrushes (Seiurus ludovicianus, S. auricapillus, and S. noveboracensis) have taken many preliminary steps to becoming as aquatic as the dipper; and tho winterwren, and even the Maryland yellow-throat arc not far behind." 1 Another curious example of the way in which species have been modified to occupy new places in nature, is afforded by the various animals which inhabit the water-vessels formed by the leaves of many epiphytal species of Bromelia. Fritz Miillcr has described a caddis-fly la.rva which lives among these leaves, and which has been modified in the pupa state in accordance with its surroundings. The pupro of caddis-flies inhabiting streams have fringes of hair on the tarsi to enable them to reach the surface on leaving their cases. But in the species inh~biting bromclia leaves there is no need for swimming, a.ncl accordmgly we find the tarsi entirely bare. In the same plants are found curious little Entomostraca, very abundant there hut found n?whcrc else. These form a new genus, but arc most nearly alhed to Cythere, a marine type. It is believed that tho transmission of this species from one tree to another m~st be ~ffe~ted by the young crustacea, which are very mmutc, clmgmg to beetles, many of which both terrestrial and. aquatic, also inhabit the bromclia leaves;' and as some watcrhcctlcs are known to frequent the sea, it is perhaps by these 1aca.ns that the first emigrants established them elves in this fitr.ln~e new abode. Bromeliro are often very abundant on trees g.r~wmg on the water's edge, and this would facilitate the tranSitiOn from a marine to an arboreal habitat. Fritz Miillcr h<lS also found, among the bromelia leaves a small froa bearin<,. 't . ' b 0 1 s eggs on Its back, and. hav~rrg some other peculiarities of stru~turc.. Several beautiful httle aquatic plants of the genus Utnculana or bladder-wort also inhabit bromelia leaves; aml these send ru?ncrs out to neighbouring plants and thus spread themselves w1th great rapidity. 1 Nature, vol. xxx. p. 30. v NATURAL SELECTION 119 1'he I rnp01·t(mre of Isolation. Isola.tion is no doubt an important aid to natural selection, as shown by the fact that islamls so often present a numl or of peculiar species; a.nd tho same thing is seen on the two sides of a great mountain range or on opposite coasts of a continent. The importance of isolation is twofold. In the first place, it leads to a body of individuals of each species being limited in their range and thus subjected to uniform conditions for long spaces of time. Both the direct action of the environment and the natural selection of such varieties only as are suited to the conditions, will, therefore, be able to produce their fnll effect. In the second place, the process of change will not be interfered with by intercrossing with other individ.uals which arc becoming adapted to somewhat different conditions in an adjacent area. But this que tion of the swamping off'ects of intercrossing will be considered in another chapter. Mr·. Darwin was of opinion that, on the whole, the largeness of the area occupied by <L species was of more importance than isolation, as a factor in the production of new species, and in this I quite agree with him. It must, too, be remembered, that isolation will often be produced in a continuous area whenever a species becomes modified in accordance with varied conditions or diverging habits. For example, a widc-mnging species may in the northern and colder pa.rt of its area become mo lificd in one direction, and in the southern part in another direction; and though for a long time an intermedia.te form may continue to exist in the intcrvmting area, this will be likely soon to die out, both beca-use its num hers will be small, and it will bo more or less pressed upon in varying season hy the modified varieties, each better able to endure extremes of climate. So, when one portion of a terrestrial species takes to a more aJ.~boreal or to a more aquatic modo of life, the change of habit itself leads to the isola.tion of each portion. Again, as will be more fully cxplai ned in a future chapter, any difference of habits or of haunts usually leads to some modification of colour or marking, as a means of concealment from enemies ; and there is reason to believe that this difference will be intensified by natural selection as a means of identification |