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ii ON THE TENDENCY OF VARIETIES, ETC. 23but the difficultyis overcome by assuming that such varietieshave strict limits, and can never again vary further from theoriginal type, although they may return to it, which, from theanalogy of the domesticated animals, is considered to be highlyprobable, if not certainly proved.It will be observed that this argument rests entirely on theassumption that varieties occurring in a state of nature arein all respects analogous to or even identical with those ofdomestic animals, and are governed by the same laws asregards their permanence or further variation. But it is theobject of the present paper to show that this assumption isaltogether false, that there is a general principle in naturewhich will cause many varieties to survive the parent species,and to give rise to successive variations departing further andfurther from the original type, and which also produces, indomesticated animals, the tendency of varieties to return tothe parent form.The Struggle for ExistenceThe life of wild animals is a struggle for existence. Thefull exertion of all their faculties and all their energies isrequired to preserve their own existence and provide for thatof their infant offspring. The possibility of procuring foodduring the least favourable seasons, and of escaping theattacks of their most dangerous enemies, are the primaryconditions which determine the existence both of individualsand of entire species. These conditions will also determinethe population of a species and ; by a careful consideration ofall the circumstances we may be enabled to comprehend, andin some degree to explain, what at first sight appears soinexplicable the excessive abundance of some species, whileothers closely allied to them are very rare.The Law of Population of SpeciesThe general proportion that must obtain between certaingroups of animals is readily seen. Large animals cannot beso abundant as small ones ;the carnivora must be lessnumerous than the herbivora ; eagles and lions can never beso plentiful as pigeons and antelopes ;and the wild asses ofthe Tartarian deserts cannot equal in numbers the horses of