OCR Text |
Show ]](3 DARWINISM CUAP. arc the species th:tt would be most likely to be so modifl rd, while others, not becoming modified, would succumb to Lho changed conditions and become extinct. The most important condition of all is, undoubtedly, that variations should occur of sufficient amount, of a , ufficicntly diverse character, and in a large number of individual , .·o us to afford ample matcri::~ls for natural selection to act upon ; and this, we have seen, does occur in most, if not in all, Ltrg<·, wide-ranging, and dominant species. From some of Lit e~<·, therefore, tho new species :.~daptcd to the changed condi tions would usually be derived; and this would especially he the case when the change of conditions was ra.ther mpid, an<l wh<'n a correspondingly rapid modification could alone save some species from extinction. But when the ch:wgc was V(·t·y gradual, then even less abundant and less widely di strilmtd species might become modified into new forms, more c ·pceia.l ly if the extinction of many of the rarer species lcfL vacant places in the economy of nature. P1·obable Origin of the Dippers. An excellent example of how a limited group of flpccics has been able to maintain itself by acl:tpt:Ltion to one of these "vacant places" in nature, is afforded by the curious little birds called dippers or water-ou:wl., forming the gr rms Cinclus and the family Cinclidre of naturali. ts. These hirds are something like small thrushes, with very short wino·s anu tail, and very dense plumage. They frequent,, exclusively, mountain torrents in the northern hemi. phere, :t n<l. olJLain their food entirely in the water, consisting, as it docs, of waLcrbeetles, caddis-worms and other inscct-larv::e, as well as numerous small fresh-water shells. These birds, althonglt not far removed in structure from thrushes and wrens, have the extraordinary power of flying under wtLter; for such, according to the best observers, is their process of diving in search of their prey, their dense and somewhat fibrous plumage retaining so much air that the water is prevented from touching their bodies or even from wetting their fe:ttbcrs to any great extent. Their powerful feet and long cmved claws enable them to hold. on to stones at the hottom, and thus to retain their position while picking up in:cct., shells, v NATURAL SELE TION 117 etc. As Lhcy frequent chiefly the most mpid and boisterous torrents, among rocks, waterfall:, and hu<re honlders, the water is never frozen over, and they arc thus <tble to live during tho severe ·t winters. Only a very few species of dipper arc known, all those of the ol<l world being so closely allied to our British bird that some ornithologists consider them to be merely local races of one species; while in North America and the northern Andes there a,rc two other species. Here then we have a bird, which, in its whole structure, shows a close affinity to the small r typical perching birds, but which h:Ls departed from all its allies in its habits and mode of life, and has secured for itself a, place in nature where it has few competitors and few enemie.. \\To may well suppo c, that, at some remote period, a bird which was perhaps the common a,nd more gcnemlised ancestor of most of our thrushes, warblers, wren., etc., had spread widely over the great northern continent, and hitd given rise to numerous varieties adapted to special conditions of life. Among these some took to feeding on the borders of cl ar streams, picking out such brv::e and molluscs as they could reach in shallow water. When food became scarce they would attempt to pick them out of deeper and deeper water, and while doing this in cold weather many would become frozen and starved. But any which possessed denser and more hairy plumage than usual, which was able to keep out the w:tter, would survive; and thus a race would be formed which wonld depend more and more on this kind of food. Th n, following np the frozen streams into the mountains, they would be able to live there durin <r the winter; and as such places afforded them much protection from enemies and ample shelter for their ne ts and young, further adaptations would occur, till the wonderful power of diving and flying under water wa.s acquired by ~t true land-bircl. That such habits might be acquired under 'stress of need is rendered highly probable by the facts stated by the wellknown American naturalist, Dr. Abbott. lie ~ays that "the water-thrushes ( cim·us sp.) all wade in water, a.nd often, seeing minute mollnsca on the bottom of the stream, plunge both head and neck beneath tho surface, so that often, for |