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Show 110 DARWINISM CHAP. by rivals, or is in process of extinction by enemies, to save itself by adopting new habits or by occupying vacant places in nature. This is the immediate and obvious effect of all the numerous examples of divergence of character whi ·h '''c have pointed out. But there is another and los· obvions result, which is, that the greater the diversity in the organisms inhabiting a country or district the greater will be the total amount of life that can be supported there. Hence the continued action of the struggle for exi. tencc will tend. to bring about more and more diversity in each area, which mn.y be shown to be the case by sever<tl kinds of ovid.cnce. As an example, a piece of turf, throe feet by four in size, wa,s found by Mr. Darwin to contain twenty species of plants, and the~ c twenty species belonged to eighteen genera and. to cigl1 t orders, showing how greatly they differed from each oth er. Farmers :find that a greater quantity of hay is obtained from ground sown with a variety of genera of grasses, clover, etc., than from similar land sown with one or two species only ; and the same principle applies to rotation of crops, plants differing very widely from ca.ch other giving the best results. So, in small and uniform island., and in small ponds of fresh wa.ter, the plants and insects, tlwngh few in number, are found. to be wonderfully varied in character. The same principle is seen in the natura.lisation of plants and animals by man's agency in distant land.s, for the speeies that thrive best and establish themselves pcemancntly arc not only very varied among themselves but differ greatly from the native inhabitants. Thus, in the N orthcrn United • itates there arc, aceording to Dr. Asa Gray, 260 natnralisccl fl ow(·ring plants which belong to no less than 162 genera; and of these, 100 genera arc not natives of the United ~tntcfi. :-lo, in Australia, the rabbit, though totally unlike any Jlativ anim:tl, has increased so much that it probably outmnn bers in in dividuals all the native mammals of the country; n.n<l in Now Zealand the rabbit and the pig have cqua.Ily nmltiplied. Darwin remarks that this "advantage of divcrsifie<Ltion of structure in the inhabitants of the same region is, in fad, tlw same n.s that of the physiological division of hlJOnr in the organs of the same body. No physiologist doubts that a v NATURAL , ELEC1'ION 111 stomach adapted to digest vegetable matter alone, or flesh n.lonc draws more nutriment from these substance.. ., 'o, in the :rcncral economy of any bn<l, the more wid ly and pcrfc~tly the animals and plants £Lrc diver ifiec~ f~r. different hn.bits of life, so will a gr atcr number of md1v1duals be capable of there supporting themselves." 1 The most closely allie,d ~Jecies inhabit distinct Areas. One of the curious results of the general action of tbi.principlo in nature is, that the m fit closely allied speciesthose whose differences though often real and important arc hardly perceptible to any one hut a, naturabst-~Lrc n uaJly not found in the same but in wi<l ly separated conntt·ic .. Thus, the ncaro. t allies to our European golden plover are found in North America, and Ea.st Asi~t; the nea.r st ally of our European jn.y i. found in .Japan, although there arc several other species of jays in \Vc.-tcrn Asia and North Africa; and though we have scvcrn.l :pecics of titmice in Enrrland they arc not very closely alli d to each other. Th~ form most akin to om· hlnc tit i. the azure tit of Central Asia (Pttrus ~tznrcns) ; the Pam 1 '<lonci of Algedn. is very near our coal tit, and the Paru. lngulJ1·is of SouthEastern Europe and Asia Minor is Jlcarest to our marsh tit. So, our four species of wild pigeons-tho ring-dove, _stockdove, rock-pigeon, and turtle-dove-arc not clo ·ely albed to each other bnt each of them belongs, according to some ornithologi~ts, to a separate genus or subgenus, and has it. nearest relatives in distant parts of Asia and Africa. In mammalia the same thing occurs. Each mountain region of Europe and Asia has usually its own species of wild shc~p and croat and sometimes of antelope and deer; so that m each 0 region there is found the gr~atc.st di_vcrsi~y i~ ~his class of animals, while the closest alhcs mhab1t qmte d1 tmct and often distant areas. In plants we find the same phenomenon prevalent. Distinct species of columbine are found in Central Europe (Agnilcgia vulgaris), in Eastern Europe, and Siberia (A. gla.ndulosa), in the Alps (A. A:Ipina), in the Pyrenees (A. pyrenaiea), in the Greek mountams (A. ottonis), and in Corsica (A. Bernardi), but rarely arc two 1 Origin of Species, p. 89. |