is
variation in the plumage. Supposing we take a dozen cock Chaffinches and
examine them carefully, we shall find slight differences in pattern and
in colour--more grey here or a duller red there, as the case may be--and
though these differences may not be sufficient to enable us to pick out
a bird at a distance, they are nevertheless conspicuous when it is close
at hand. Then again there is variation in the song; and the more highly
developed the vocal powers the greater scope there is for variation. But
even the phrases of a simple song can be split up and recombined in
different ways. If one were asked casually whether the different phrases
of the Reed-Bunting's song always followed one another in the same
sequence, the answer would probably be that they certainly did so,
whereas the bird is capable of combining the few notes it possesses in a
surprising number of different ways. And lastly, there are differences
in just the particular way in which specific behaviour, founded upon a
congenital basis, is adapted by each individual to its own special
environment. Racial preparation determines behaviour as a whole, but the
individual is allowed some latitude in the execution of details which
are in themselves of small moment--the selection of a particular tree as
a headquarters and a particular branch upon that tree, the direction of
the distant excursion, and the direction of the limited wanderings
within the small area surrounding the headquarters which in the course
of time determine the extent of the territory, are matters for each
individual to decide when the occasion for doing so arises. Moreover
instances of abnormal coloration or abnormal song are not rare, and they
are valuable since they place the identity of the individual beyond
dispute. I can recall the case of a Willow-Warbler whose song was unlike
that of its own or any other species, and of a Redbreast whose voice
puzzled me not a little. I can recollect also a male Yellow Bunting
whose foot was injured or deformed. Of this bird's behaviour I kept a
record for two months or so; and inasmuch as it inhabited a roadside
hedge, and was of fearless disposition, the deformed foot could plainly
be seen whenever it settled upon the road to search for food.
Identification is not, therefore, a difficulty. There is always some
small difference in colour or in song, or some well-defined routine
which makes recognition possible.

Owing to their great powers of locom