166 THE WORLD OF LIFE 



sects but what has its more or less dangerous enemies among 

 the birds. Even the great rapacious birds, the hawks, buz- 

 zards, and owls, when their special food, the smaller mammals 

 and birds, fails them, will capture almost every kind of 

 ground-feeding insects; while the enormous tribes which feed 

 largely on frnits and seeds often make up for its deficiency 

 by capturing such insects as are available. 



One of the clearest deductions from these facts is, that the 

 great variety of the smaller birds — warblers, stonechats, tits, 

 w^agtails, pipits, wrens, and larks — owes its origin to the 

 continuous specialisation throughout the ages of new forms of 

 birds adapted to take advantage of every fresh development 

 of the insect tribes as they successively came into existence. 

 As Darwin repeatedly impresses upon us, excessive powers of 

 multiplication with ever-present variations, lead to the almost 

 instant occupation of every vacant place in the economy of 

 nature, by some creature best fitted to take advantage of it. 

 Every slight difference in the shape or size of bill, feet, toes, 

 wing, or tail, or of colour of the various parts, or of supe- 

 rior acuteness in anv of the senses, such as we can see in the 

 different allied species of these birds, has been sufficient to 

 secure the possession of some one of these vacant places ; and 

 when this first partial adaptation has been rendered more and 

 more perfect by the survival in each successive generation of 

 those individuals best fitted for the exact conditions of the 

 new environment, a position is reached which becomes at any 

 future time a secure starting-point for further modification, 

 either in the same or in any slightly diverging line, so as to 

 be again fitted to occupy some other vacant place which may 

 have arisen through the slightest changes either in the inor- 

 ganic or the organic environment. 



So long as we limit ourselves to a consideration of the mode 

 in which any existing species has been produced, by the 

 adaptive modification of some other pre-existing closely allied 

 species, by means of the known facts of universal variation 

 and of the constant survival of the best adapted, there is no 



