ISOLATION (from isolate, from Fr. isoler, It. isolate, from ML. insulare, to separate, from Lat. insithi, island, from in, in &a/nil, sea. Gk.
aciXos, surge). In evolution, the separa tion or segregation of any assemblage of plants or animals in a limited area, so that the incipient varieties or species are prevented from breeding with those of adjoining regions. Through such isolation the leveling effects of free crossing or mixing with allied varieties is prevented. 1 hug variations or nascent species 'become localized, with the result that there are many thousands of local races, varieties, and species.
Besides geographical isolation, there are other kinds of segregation. Darwin suggested two forms: (1) Arising from organisms breeding at slightly different seasons; (2) "from varieties of the some kind preferring to pair together." To the first of these may be added the inbreeding of butterflies of two different broods, a part of one brood being belated and flying with their 'nephews and nieces.' See Lamarck was the first to broach the subject of the doctrine of isolation as a factor in species making in referring to maw Considering organ isms in general, he points out that in reproduc tive unions the crossings 'between the individuals which have different qualities or forms are neces sarily opposed to the continuous propagation of these qualities and their forms. He then instances man, and says that, if distance of habitation did not separate men, the intermixture by generation would cause the general elmraeteristies distin guishing different nations to disappear. Wagner (1868) has fully proved by numerous examples the importance of migration and isolation in spe cies-making. See :NIP:RATION, WAGNER'S LAW OF As a result of fifteen years' collecting in the Hawaiian Islands of land shells belonging mostly to genera found nowhere else, Gulick established the fact that in each mountain valley of the forest region of Oahu there is a great number of local species (200, represented by 700 or SOO varieties) belonging to several genera, and that each of the twenty valleys contains one or more local varieties or species restricted to that valley. On tracing this wonderfully differentiated assem bly from valley to valley, it became apparent that a slight variation in the occupants of a valley as compared with those of the adjacent valley be comes more pronounced in the next or third valley, still more in the fourth, and so on. Thus, Ise was able roughly to the amount of di vergence between the occupants of any two given valleys by measuring the number of miles be tween them (Romanes). Guliek thinks the evo lution of these different forms cannot be attribut ed to differences in their external conditions. The forest area, covering one of the mountain ranges, in which the snails live, is about 40 miles long and 5 or G miles wide. Ile states that the rainfall on the northeast side of the mountain is somewhat heavier than on the opposite side, and the higher ridges of the mountains are cooler than the valleys; but the valleys on one side of the range have a climate the same in every respect.
The vegetation in the valleys differs somewhat from that on the ridges; hut the vegetation of the different valleys is much the same; the birds, in sects, and larger animals are the same. Though, as far as we can observe, the conditions are the same in the valleys on one side of the range, each has a molluscan fauna differing in some degree from that of any other. lie also adds that a genus is represented in several successive valleys by allied species. sometimes feeding on the same, sometimes on different plants. In every such case it appeared that the valleys that are nearest to each other furnish the most nearly allied forms, and a full set of the varieties of each species pre sents a minute gradation of forms between the more divergent types found in the more widely separated localities. After giving reasons tor the belief that this variation is not due to dit ferences in their external conditions. Gulick con cludes that the difference is due to a corre sponding difference in the time of separation of each variety. and also to what he terms 'cumula tive segregation,' segregate breeding,' and 'inde pendent generation.' In his hiteresting essay on Physiological Selection (188(1 and 1897), Ttomanes states that the essence of the principle consists in all cases of the diversifying effect of cross-infertility, whensoever and howsoever it may happen in particular eases to have been caused. (See PHYSIOLOGICA L SELECTION.) Tt is to he observed that this is but little different from Guliek's 'cumulative' or 'intensive' segre gation. See I.:you-Dux, paragraph !'actors of Erolution CLAssuewArloN OF ANIMALS. para graph 8pecies.
After all the careful work clone by Gullet:. it is yet to be doubted whether the chief or initial factors in the wonderful specialization which has taken place in the land shells of Dahu are not the result of migration into new regions, vary ing in natural conditions, For the present, then, Wagner's factors of migration into areas with differing conditions of life, and isolation. and the consequent prevention of intererossing with the criginal or parent forms, may he accepted as the essential causes of the origin of perhaps two thirds one-half of existing as well as extinct species.
It! IILIOGRA PH Y. Ueter die Darwinisehe Theorie in Bezug aid rlic gcographische Icrbreitung der Organismen (Munich, 186S) ; The Darwinian Theory and the Law of the Migration of Organ isms (Leipzig, 1868; trans. by Laird, London, 1873) ; Leber den L'influss der grographischen lsoli•ung and Colonictibihlung auf die nco•pho laqisc/•n l'criimlerungen der O•ganismen (Mu nich, 1570); Gulick, "On Diversity of Evolu tion Under One Set of External Conditions," in .lournal of the Linnean Nociely (London. 1872) ; Dirergent Erolution Through Cumulatire tiegrc gation (ib.. 1888) ; "Divergent Evolution and the Darwinian Theory," in Jmeriran -lournal of Science (New Haven, .1aunutry. 1890) ; "inten sive Segregation. or Divergence Through Inde pendent Transformation," in Journal of the Liccnean Society (London, 1890 ) ; noma nes, "Physiological Selection," in Journal of the Linnean Society (ib.,