CHAPTER XV 
CLASSIFICATION 
STRICTLY speaking, classification is not part of the 
subject of this book. But since it is based on 
structure, it cannot be entirely passed over. And, in 
fact, it cannot be properly studied without a great 
deal being learnt beyond the distinguishing marks of 
species, genera, and families. 
The aim of the classifier is to discover the relation- 
ship of bird to bird and arrange them in natural 
groups. Ifthe system be one that enables the learner 
easily to identify a specimen, so much the better, but 
‘that is not the object in view. The term relationship 
has gained a far more definite meaning, since the theory 
of evolution has been generally adopted. If two 
animals are described as related to one another, the 
meaning is that they are descended from the same 
ancestors. If taken in the broadest sense, this is a 
mere truism, since it is held that all species have been 
developed from one. It must be understood to mean 
that, if the two lines of descent be followed upward, 
