154 THE WORLD OF LIFE 



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, stone- 

 chats, tits, wagtails, 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 varia- 

 tions, 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 superior acuteness in any 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 inorganic 

 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 difficulty whatever in accepting the " origin of 

 species " from other species as a demonstrated fact ; and 

 this alone was the hitherto insoluble problem which Darwin 

 first succeeded in solving. It is only in the extension of 

 the process to isolated groups such as the whales, the 

 elephants, the serpents, or the mammalia ; or by enquiring 

 how special organs, such as horns, teeth, ears, or eyes, could 

 have begun their process of development, that difficulties 



