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Show 330 GEOGRAPHICAL DISTRIBUTION, [CB.AP, XI. loO'ical evidence that the whole body of arctic shells underw~ nt scarcely any modification during their long southern migration and re-migrati.on northw.ard, t~e case may h~ve been wholly different With those Intruding forms wlnch settled themselves on the intertropical mountains, and in the southern hemisphere. These b~ing surrounded by strangers will have had to compete With many new fonns of life; and it is p~obable that .sel~cted ~odifications in their structure, habits, and constitutions Will have profited them. Thus many of these wanderers, though still plainly related by inheritance to their brethren of the northern or southern hemispheres, now exist in their new homes as well-marked varieties or as distinct species. It is a remarkable fact, strongly insisted on by !-looker in regard to America, and by Alph. de Candolle in regard to Australia, that n1any more identical plants and allied for1ns have apparently migrated from the north to the south, than in a reversed direction. We see, however, a few southern vegetable forms on the mountains of Borneo and Abyssinia. I suspect that this preponderant migration from north to south is due to the greater extent of land in the north, and to the~ northern forms having existed in their own homes in greater numbers, and having consequently been advanced through natural selection and competition to a higher stage of perfection or dominating power, than the southern forms. And thus, when they became commingled during the Glacial period, the northern forms were enabled to beat the less powerful southern for1ns. Just in the same manner as we see at the present day, that very many European productions cover the ground in La Plata, and in a lesser degree in Australia, and have to a certain extent beaten the natives; whereas extremely few southern forms have become naturalised in any part of Europe, though hides, wool, and other objects likely to carry seeds, have been largely imported into Europe during the last two or three centuries from La Plata, and during the last thirty or forty years from Australia. Something of the same kind must have occurred on the intertropical mountains : no doubt before the Glacial period they were stocked with endemic Alpine OIU.P. XI.] DURING THE GLACIAL TERIOD . 331 forms; but these have al t to the more dominant formnlos everywhere lar1ely yielded and more effic.i ent 'vorkshos , g efn ehra t ed I· n the arger areas ands the native productio ps ~ t e north. In many isl-t b d b ou num ere y the naturanlsi saerdc .n earlJ.r e qua1 1 ed or even not been actually exterminated tl a?-d If the natives have ~rea~ly reduced, and this is tbe lfl~r numbers have been tinction. A mountain is a . 1 rst stage towards ex-intertropical mountains be~o~~e a~g on th~ land ; and the have b~en conlpletely isolated. e Glamal. period must productions of these islands th an1d I b~hevc that the produced within the larger aon e f and Yielded to those flame way as the productionsr~~s o lt~el north, just in the where lately yielded to cont' {1a :DIs ands have everyman's agency. Inen a orms, naturalised by I am far from supposinO' tl . moved on the view here O'iv 5 • 1at all difficulties are re-a: ffinities of the allied sp~cie~n;h. r~grrd t? the range and and southern temperate zon Ic IVe In the northern the intertropical regions Vs and on t~e mountains of to be solved. I do not p.retm ~·~ ~a~;y drfficulties remain and means of migration or ~h o In wate the exact lines cies and not others hav~ . e reason why certain spehave been modified and h migi:ated; why certain species forms, and others have re~=·gi~en risle to new_groups of h.ope to explain such facts u~~~ una tered. We cannot ~Ies and .not another beco~es nat we l?andsbay why one spem a foreign land. wh on ura Is.e y man's agency a~d _is twice or 'thri!e ase ~~r:ffes twice or thrice as far, Within their own homes mon, as another species I have said that rna . d'ffi I . some of the most re n) I cu ties rmnain to be solved. clearness by Dr. I-Io~~I~ai~e ~re stat~d with admirabl~ antarctic reO'ions Tb his botaniCal works on the ~ill only s~y that as ef:r c:~not be here discussed. I Identical species at point regards the occurrence of guelen Land New z 1 s dso enormously remote as ICer-to war ds the ' close ofe at haen Gal nd. FI uegi.. a, I b. eh.e ve that dl%gested b_y Lyell have been fCia I period, Icebergs, as lSpersal. But th~ ex. t afrge y concerned in their IS ence o several quite distinct |