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

;48 DISTRIBUTION OF ANIMALS. [part i.ground against the rival forms whose numbers were intact.Some would probably diminish and rapidly die out; otherswhich produced favourable varieties, might be so modified bynatural selection as to maintain their existence under a differentform ; and such changes would take place in varying modes onthe two sides of the new strait.6. But the progress of these changes would necessarily affectthe other species in contact with them. New places would beopened in the economy of nature which many would struggleobtain ; and modification would go on in ever-widening circleand very long periods of time might be required to bring thewhole again into a state of equilibrium.7. A new set of factors would in the meantime have comeinto play.The sinking of land and the influx, of a large bodyof water could hardly take place without producing importantclimatal changes.The temperature, the winds, the rains, mightall be affected, and more or less changed in duration and amount.This would lead to a quite distinct movement in the organicworld.Vegetation would certainly be considerably affected, andthrough this the insect tribes. We have. seen how closely thelife of the higher animals is often bound up with that of insectsand thus a set of changes might arise that would modify thenumerical proportions, and even the forms and habits of a greatnumber of species, would completely exterminate some, and raiseothers from a subordinate to a dominant position. And all thesechanges would occur differently on oppositetosides of the strait,since the insular climate could not fail to differ considerablyfrom that of the continent.8. But the two sets of changes, as above indicated, producedby different modes of action of the same primary cause, wouldact and react on each other ;and thus lead to such a far-spreadingdisturbance ofthe organic equilibrium as ultimately perhapsto affect in one way or another, every form of life upon theearth.This hypothetical case is useful as enabling us better to realizehow wide-spreading might be the effects of one of the simplestchanges of physical geography, upon a compact mass of mutually