The geographical distribution of animals, with a study of the relations ...

—46 DISTRIBUTION OF ANIMALS. [part i.adapted to live, to increase, and to maintain itself under adversecircumstances, than the other.Now if we consider carefully the few suggestive facts herereferred- to (and many others of like import are to be found inMr. Darwin's various works), we shall be led to conclude thatthe several species, genera, families, and orders, both of animalsand vegetables which inhabit any extensive region, are boundtogether by a series of complex relations ;so that the increase,diminution, or extermination of any one, may set in motion aseries of actions and reactions more or less affecting a largeportion of the whole, and requiring perhaps centuries of fluctuationbefore the balance is restored. The range of any speciesor group in such a region, will in many cases (perhaps in most)be determined, not by physical barriers, but by the competitionof other organisms. Where barriers have existed from a remoteepoch, they will at first have kept back certain animals fromcoming in contact with each other; but when the assemblageof organisms on the two sides of the barrier have, after manyages, come to form a balanced organic whole, the destructionthe barrier may lead to a very partial intermingling of thepeculiar forms of the two regions. Each will have becomemodified in special ways adapted to the organic and physicalconditions of the country, and will form a living barrier to theentrance of animals less perfectly adapted to those conditions.Thus while the abolition of ancient barriers will always leadtomuch intermixture of forms, much extermination and widespreadalteration in some families of animals ; other importantgroups will be unable materially to alter their range ;or theymay make temporary incursions into the new territory, and beultimately driven back to very near their ancient limits.In order to make thissomewhat difficult subject more intelligible,it may be well to consider the probable effects of certainhypothetical conditions of the earth's surface :1. If the dry land of the globe had been from the firstcontinuous, and nowhere divided up by such boundaries as loftymountain ranges, wide deserts, or arms of the sea, it seemsprobable that none of the larger groups (as orders, tribes, orof;