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Show 58 ISLAND LTF E. [PAUl'. T. question of differences of age or sex, and the pair last referred to were taken at the same place and on the same day.l These interesting facts entirely support the belief in the variability of all animals in all their parts and organs, to an extent amply sufficient for natural selection to work with. We may, indeed, admit that these are extreme cases, and that the majority of species do not vary half or a quarter so much as shown in the examples quoted, and we shall still have ample variation for all purposes of specific modification. Instead of an extreme variation in the dimensions and proportions of the various organs of from 10 to 25 per cent. as is here proved to occur, we may assume from 3 to 6 per cent. as generally occurring in the majority of species; and if we further remember that the above excessive variations were found by comparing a number of specimens of each species, varying from 50 to 150 only, we may be sure that the smaller variations we require must occur in considerable numbers among the thousands or millions of individuals of which all but the very rare species consist. If, therefore, we were to divide the population of any species into three groups of equal extent, with regard to any particular character-as length of wing, or of toes, or thickness or curvature of bill, or strength of markings-we should have one group in which the mean or average character prevailed with little variation, one in which the character was greatly, and one in which it was little, developed. If we formed our groups, not by equal numbers, but by equal amount of variation, we should probably find, in accordance with the law of averages, that the central group in which the mean characteristics prevailed was much more numerous than the extremes, perhaps twice, or even three times, as great as either of them, and forming such a series as the following- 10 maximum, 30 mean, 10 minimum developm~nt. In ordinary ca,ses we have no reason to believe that the mean characters or the amount .of variation of a species changes materially from year to year or from century to century, and we may there- 1 These facts nre taken from a memoir on The Mammals and TVinter Birds of lPlorida, by J. A. Allen ; forming Vol. II., No. 3, of the Bulletin of the Museum of Comparative Zoology at Harvard College, Cambridge, Massachusetts. CliAP. Ir.] EV 1 OL UTI ON THE KEY TO DISTRIBUTION. 5!) fore look u th . b pon e central group as the type of the species which Is . est ada?ted to the conditions in which it has actually to exist. This t .11 th r ype WI ere1ore always form the majority be-cause the str 1 £ . . ' su . ugg c or existence Will lead to the continual t . ppresswn of the less perfectly adapted extremes But some Imes a species h ·a · · - . . . as a WI e range mto countries which differ in pFy~cal conditiOns, .and then it often happens that one or other o t e extremes Will. p~edominate in a portion of its rancre. These form local vaneties, but as they occur mixed with tlle other forms, they are not considered to be distinct s· . although the~ may differ from the other extreme form q~~~~e~~ much as species often do from each other. How new Species arise frorn a variable Species.-It is now very easy to understand how, from such a variable species one or m?~e new species may arise. The peculiar physical or or~anic cond1t10ns that render one part of the area better adapted to an ext:e~e form may become intensified, and the most extreme vanatwns thus having the advantage, they will multiply at the expense of the rest. .If the change of conditions spreads over t~e whole area occupied by the species, this one extreme form wlll re~lace the others; while if the area should be cut in two by subs~denc~ or elevation, the conditions of the two parts may be mod1fied m oppo~ite d~rections, so as to be each adapted to one. extrem~ form; m whiCh case the original type will become e~tm~t, bemg r~placed by two species, each formed by a com_ ?matwn of certam extreme characters which had before existed m some of its varieties. ~h~ changes of conditions which lead to such selection of vanet1es are very diverse in nature, and new species may thus be for~ed, diverging in many ways from the original stock. The chmate may change from moist to dry, or the reverse, or t~e temperature may increase or diminish for loner periods in either c~se requiring a corresponding change of ~onstitution, of co;ermg, of. vegetable or of insect food, to be met by the s:lectwn of vanations of colour or of swiftness, of length of bill or ~f strength of claws. Again, competitors or enemies ma! a.rnve from other regions, giving the advantage to such vanet1es as can change their food, or by . swifter flight or |