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Show CHAPTER IV. EVOLUTION THE KEY TO DISTRIBUTION· t · of EvoI ut.w n-The Origin of New ASp ecie·~ :~- Importance of the Doc nne t of variation in North menctm Variation in Animals-The. amoun varhble species-Definition and Birds-How new speC·i es anse froll1 t'an ction• of Species-'I,h e n·s e an d On. gm. of G enei· a- Cause of thDe' ex n1i inuous speCI' fic m.e as, w1 . ·e- 1Y Jar decay of Species and Genera- lSCO 1 tria-Discontinuity of Emberiza ' f h of Parus pa us 1 £ Discontinuity o t e area d J ese Jays- Supposed examp es o schooniclus-The European an .apanB. ds-Distribution and antiquity . . North Amencan 1r . doifs Fcoanmtmilimesty-D aimscoonngt m. m. ty a proo f of autiquity-Concludmg Reruar 1-: s. . h ters we have explained the general nature IN the precedmg c ap t d b the distribution of animals, and of the. phenomena pr~:::d t~ new geographical division of the have Illustrated and 'th them Before wo go h h' h . s found best to agree WI . eart w IC I . f bject and especially before we further into the details o ou~· s~ hav: brought about the exist-t t to trace the causes w IC l t J ~t e~p . . f the islands of the globe, it is abso u e y mg biological relatlOns o h 'on of the collateral facts t b e a clear compre ensi necessary ·O a~ . to which we shall most frequently have and general prmCiples b b . fly defined as the powers · t fer These may e ne ' occa~lOn ore f . . I nd plants under different conditionsof dispersal o amma s a d the oriO'in and develop-geological and climatal changes-·anl I t' o This last is of . d s by natura se ec 10n. ment of species an gr?up d . t bearinO' on the dispersal the most fundamentalimporta~ce~ a; I~e ther~fore devote the of animals has been much .neg e? e . present chapter to its conside~atlOn. fi t chapter that the distriAs we have already shown In o~r ;s families present almost bution of species, of genera, an o ' CHAP. Iv.] EVOLUTION 'l'HE KEY TO DISTRIBUTION. 55 exactly ~he same genera] phenomena in varying degrees of complexity, and that almost all the interesting problems we have to deal with depend upon the mode of dispersal of one or other of these; and as, further, our knowledge of most of these groups, in the hiO'her animals at least is confined to the 0 ' tertiary period of geology, it is therefore unnecessary for us to enter into any questions involving the oriO'in of more comprehensive groups, such as classes or orders. 0 This enables us to avoid most of the disputed questions as to the development of animals, and to confine ourselves to those general principles regulating the origin and development of species and genera which were first laid down by Mr. Darwin twenty years ago, and have now come to be adopted by naturq.lists as established propositions in the theory of evolution. The Origin of New Species.-How, then, do new species arise, supposing the world to have been, physically, much as we now see it; and what becomes of them after they have arisen? In the first place we must remember that new species can only be formed when and where there is room for them. If a continent is fully stocked with animals, each species being so well adapted for its mode of life that it can overcome all the dangers to which it is exposed, and maintain on the average a tolerably uniform population, then, so long as no change takes place, no new species will arise. For every place or station is supposed to be filled by creatures perfectly adapted to all surrounding conditions, able to defend themselves from all enemies, and to obtain food notwithstanding the rivalry of many competitors. But such a perfect balance of organisms nowhere exists upon the earth, and probably never has existed. The well-known fact that some species are very common, while others are very rare, is an almost certain proof that the one is better adapted to its position than the other; and this belief is strengthened when we find the individuals of one species ranging into different climates, subsisting on different food, and competing with different sets of animals, while the individuals of another species will be limited to a small area beyond which they seem unable to extend. When a change occurs, either of climate or geography, some of the small and ill-adapted species |
