THE ORIGIN OF SPECIES AND GENERA 291 



varying conditions which occur during considerable cycles 

 of climatal and other changes, the balance will have 

 become well established, and so long as no change takes 

 place in the conditions no new species will arise. 



But now let us suppose some change to begin, either of 

 climate or geography. The land may sink or it may be 

 elevated, in the former case diminishing in area and per- 

 haps becoming divided by an arm of the sea, in the latter 

 case increasing in area and perhaps becoming united with 

 extensive lands formerly separated from it ; or the climate 

 may become moister or drier, hotter or colder, more 

 extreme or more equable, and any one of these changes 

 or any combination of them would, it is easy to see, pro- 

 duce a special effect on the forms of life. The vegetation 

 would in almost any case become changed, and this would 

 affect both the insects and the higher animals in a variety 

 of ways. New enemies or new competitors might be 

 admitted, and these would certainly cause the extermina- 

 tion of some of the rarer species, and perhaps greatly 

 reduce the numbers of those which had been most 

 numerous. Others might, from the same general causes, 

 obtain fresh supplies of food, or have fresh areas over 

 which to spread themselves opened to them. 



These are the first and most obvious effects of such 

 changes, but there are others still more important, and 

 not less certain to be produced. We have supposed each 

 of the species which inhabited the country to be well 

 adapted to the conditions of its existence, to be able to 

 obtain food for itself and young, to protect itself against 

 all kinds of enemies, and to be able to resist the ordinary 

 inclemencies of the seasons, and to do all this in com- 

 petition with the numerous other species by which it 

 was surrounded. But now, we suppose, all these con- 

 ditions and surroundings to be undergoing change, either 

 at the same time or in succession; and, in order to 

 become equally well adapted to the new conditions, some 

 of the species will require to undergo a corresponding 

 change, either in structure, habits, colour, or some other 

 characters. New enemies may necessitate greater swift- 

 ness, or greater cunning, or less conspicuous colours ; less 



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