ore 
INTRODUCTION. 
Tue 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Yarrell’s ‘British Birds’ are quite out of date.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 of the history 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 development 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 affirmative, the study of ornithology is found 
to possess 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 lines must be devoted to 
Tur Hyroruesis or Evonurion. 
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 off, 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 
