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ii ON THE TENDENCY OF VARIETIES, ETC. 33rendering existence difficult and extinction almost sure soonto follow. An origin such as is here advocated will alsoagree with the peculiar character of the modifications of formand structure which obtain in organised beings the manylines of divergence from a central type, the increasing efficiencyand power of a particular organ through a succession ofallied species, and the remarkable persistence of unimportantparts, such as colour, texture of plumage and hair, form ofhorns or crests, through a series of species differing considerablyin more essential characters. It also furnishes us witha reason for that " more specialised structure " which ProfessorOwen states to be a characteristic of recent comparedwith extinct forms, and which would evidently be the resultof the progressive modification of any organ applied to aspecial purpose in the animal economy.ConclusionWe believe we have now shown that there is a tendencyin nature to the continued progression of certain classes ofvarieties further and further from the original type a progressionto which there appears no reason to assign anydefinite limits and that the same principle which producesthis result in a state of nature will also explain why domesticvarieties have a tendency, when they become wild, to revertto the original type. This progression, by minute steps, invarious directions, but always checked and balanced by thenecessary conditions, subject to which alone existence can bepreserved, may, it is believed, be followed out so as toagree with all the phenomena presented by organisedbeings, their extinction and succession in past ages, and allthe extraordinary modifications of form, instinct, and habitswhich they exhibit.