INTRODUCTION. 



The number of books which have been published on British birds is so 

 great that it might be thought that every thing that could be said on the 

 subject had been already well said. But such is the rapid progress 

 which ornithology has made during the last few years that even the 

 earlier portions of Dresser's ' Birds of Europe ' and Newton's edition of 

 Yarrell's ' British Birds ' are quite out of date. Not only have many 

 important gaps in the geographical distribution of some of our commoner 

 birds been filled up, and a large part o£ the histoiy of some of the rarer 

 ones been discovered, but in many respects I have found it necessary to 

 look upon the whole subject from a different point of view. The argu- 

 ments in favour of the theory that the species of animals now existing in 

 the world were evolved by natural laws, some of which we have discovered, 

 from species of a more primitive type which lived in remote geological 

 ages are so irresistible that it is impossible to ignore them. At the 

 first glance it would seem that the developmeut of a species was a subject 

 quite apart from its present history ; but it will be found that this 

 question of the development of species by evolution is one which lies at 

 the foundation of all inquiries into the history of individual species ; and 

 when it is answered in the aiErmative, the study of ornithology is found 

 to j)ossess a new interest, many obscure points become comparatively 

 clear, and the old treatment of the subject requires modifying in various 

 ways. It is of the utmost importance to have clear ideas on this subject, 

 in order rightly to interpret the facts of Nature ; and consequently a 

 few liaes must be devoted to 



The Hypothesis or Evolution. 

 There is amongst birds, as there is throughout the animal and vegetable 

 world, a more or less keen " struggle for existence." The natural increase 

 is so rapid that the surplus population is necessarily killed ofi', partly by 

 falling a prey to stronger animals, partly by want of food, partly by disease, 

 and partly, especially in the case of migratory birds, by other forms of 

 violent death. Consequently we find that a weeding process is constantly 

 going on throughout Nature. The weak die; the strong live: the fit 

 survive; the unfit perish. This is called the " survival of the fittest." But 



