MIGRATION, NVAGNEWS LAW OF. in evolu tion. one of the main agencies leading to the isolation of animals, and consequently- to the for mation of local 'caws. varieties. and species. This was first pointed out by Moritz Wagner in 1S6s.
stated his views in three general propositions: (1) The greater the change in the conditions to which individuals are subjected on emigrating to another territory, the more intense must be the inherent individual variability of each organism; (2) the less the even tenor of this increased in dividual variability of organisms is interrupted by frequent crosses with emigrants of the old stock, the more frequently will nature be success ful in forming a new variety or incipient species by the acememlation and inheritance of fresh char acteristics: (3) the more advantageous to the variety the change in each single organ, the bet ter it will he able to adapt itself to surrounding ciremnstanees; and the longer the selection of an incipient variety of colonists remains undis turbed by the old stock, the more frequently will a new species arise out of the variety. These laws are fully illustrated by Wagner in his works, and by later observers. Wagner claimed
that evolution by natural selection is impossible. unless it be assisted by geographical isolation. in order to prevent the swamping effects of inter crossing. The numerous facts observed by Wag ner and others show that some of the barriers isolating incipient species are broad rapid rivers, oceans, and mountains. lle speaks of the ex traordinary phenomena of so many species of plants and animals peculiar to the volcanoes and isolated mountains of Quito. "Without the dis tribution of organisms by migration in connect ion with local selection it would be inexplicable. The gigantic isolated mountains of this highland act a similar part in the formation of varieties and species as the islands of the archipelago, for instance, of the t :al:limp-is." Wagner also claimed that adaptation to changed conditions of life and transformaC appear to he tantamount to a renovation. Species, he said. whirl did not mi grate, and did not alter in form.
IN•eallIP 1'0 NVagner, The Th, c,ru u nil th,/sere of the II igra t ion of Organ isms (Mimic)). 18118; trans. by Laird, London, 1 ..+73 .