48 DISTRIBUTION OF ANIMALS. [part i. 



ground against the rival forms whose numbers were intact. 

 Some would probably diminish and rapidly die out; others 

 which produced favourable varieties, might be so modified by 

 natural selection as to maintain their existence under a different 

 form ; and such changes would take place in varying modes on 

 the two sides of the new strait. 



6. But the progress of these changes would necessarily affect 

 the other species in contact with them. New places would be 

 opened in the economy of nature which many would struggle to 

 obtain ; and modification would go on in ever-widening circle 

 and very long periods of time might be required to bring the 

 whole again into a state of equilibrium. 



7. A new set of factors would in the meantime have come 

 into play. The sinking of land and the influx, of a large body 

 of water could hardly take place without producing important 

 climatal changes. The temperature, the winds, the rains, might 

 all be affected, and more or less changed in duration and amount. 

 This would lead to a quite distinct movement in the organic 

 world. Vegetation would certainly be considerably affected, and 

 through this the insect tribes. We have. seen how closely the 

 life of the higher animals is often bound up with that of insects ; 

 and thus a set of changes might arise that would modify the 

 numerical proportions, and even the forms and habits of a great 

 number of species, would completely exterminate some, and raise 

 others from a subordinate to a dominant position. And all these 

 changes would occur differently on opposite sides of the strait, 

 since the insular climate could not fail to differ considerably 

 from that of the continent. 



8. But the two sets of changes, as above indicated, produced 

 by different modes of action of the same primary cause, would 

 act and react on each other ; and thus lead to such a far-spread- 

 ing disturbance of the organic equilibrium as ultimately perhaps 

 to affect in one way or another, every form of life upon the 

 earth. 



This hypothetical case is useful as enabling us better to realize 

 how wide-spreading might be the effects of one of the simplest 

 changes of physical geography, upon a compact mass of mutually 



