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Show 56 ' ISLAND LIFK [PART J, will probably die out altogether, and thus leave room for others to increase, or for new forms to occupy their place~. . . B t the chancre will most likely affect even flounshmg species in d~fferent wa;s, some beneficially, others injuriously. Or, again, it may affect a great man.y inju:iously, to such ~n extent as to require some change m theu structure or habits to enable them to get on as well as before. Now " variation " and the "struggle for existence " come into play. All_ the weaker and less perfectly organised individuals die out, while those which vary in such a way as to bring them into more harmony with the new conditions constantly survive. If the change of conditions has been considerable, then, after a few centuries, or perhaps even a few generations, one or more new species will be almost sure to be formed. Variation in Animals.-To make this more intelligible to those who have not considered the subject, and to obviate the difficulty many feel about "favourable variations occurring at the right time," it will be well to discuss this matter a little more fully. Few persons consider how largely and universally all animals are varying. We know, however, that in every generation, if we could examine all the individuals of any common species, we should find considerable differences, not only in size and colour, but in the form and proportions of all the parts and organs of the body. In our domesticated animals we know this to be the c:tse, and it is by means of the continual selection of such slight varieties to breed from that all our extremely different domestic breeds have been produced. Thiuk of the difference in every limb, and every bone and muscle, ami probably in every part, internal and external of the whole body, between a greyhound and a bull-dog! Yet, if we had the whole series of ancestors of these two breeds before us, we should probably find that in no one generation was there a greater difference than now occurs in the same breed, or sometimes even the same litter. It is often thought, however, that wild species J.o not vary sufficiently to bring about any such change as this in the same time · and thoucrh naturalists are well aware that this ' 0 is a mistake, it is only recently that they are able to adduce positive proof of their opinion. CiEIAr. IV.] EVOLUTION THE KEY TO DISTRIBUTION. 57 The A mo'unt ,-F Vi . . . - A . oJ ct?·ur-twn ~n North American BiTds.-An meriCan natu r t M ob servatw. ns anJr.a IS , r. J. A. Allen, ha. s made elaborate S measurements of the bu·ds of the United tates, and he fi d n s a wonderful anu altocrether unsuspected amount of variation b t · d' · o Th . . . ' e ween m lVIduals of the same species. b ~y differ In the general tint, and in the markin<rs and distri-utw? ~f th~ co~ours; in size and proportions; in° the length of the ~mbs, tail, b1ll, and feet; in the length of particular feathers altenng the shape of the wing or tail; in the length of the tarsl and of the separate :oes, and in the length, width, thickness, and curvature of the bill. These variations are very considerable often reaching to one-sixth or one-seventh of the averacr~ di~ensi?ns, and so~eti~es more. Thus Turdus f~tscesce~s (Wilsons thrush) vaned m length of wing from 3·58 to 4·16 inches, and in the tail from 3·55 to 4·00 inches; and in twelve specimen__s, all taken in the same locality, the wing varied in length from .14_'£> t.o ~1 per cent., ~nd the tail from 14 to 22·.? per cent. In s~aha s~ahs (the blue-bird) the middle toe varied from ·77 to ·91 inch, and the hind toe from ·58 to ·72 inch, or more than 21·5 per cent. on the mean, while the bill varied from ·45 to ·56 inch in length, and from ·30 to ·38 inch in width, or about 20 per cent. in both cases. In DendTCeca co?·onata (the yellowcrowned warbler) the quills vary in proportionate length, so that the 1st, the 2nd, the 3rd, or the 4th, is sometimes longest, and a similar variation of the wing involving a change of proportion between two or more of the feathers is recorded in eleven species of birds. Colour and .marking vary to an equal extent ; the dark streaks on the under surface of Melospiza melodia (the American song-sparrow) being sometimes reduced to narrow lines, while in other specimens they are so enlarged as to cover the greater part of the breast and sides of the body, sometimes uniting on the middle of the breast into a nearly continuous patch. In one of the small spotted wood-thrushes, Turd'ltS fuscescens, the colours are sometimes very pale, and the markings on the breast reduced to indistinct narrow lines, while in other specimens the general colour is · much darlcer, and the breast markings dark, broad, and triangular. All the variations here mentioned occur between adult males, so that there is no |