Xv INTRODUCTION. 
structural characters are most affected by the nature of the food and the 
necessity or otherwise to migrate. It will at once be seen that the former 
set of causes are much more constant than the latter in the Palearctic 
Region. There is no reason to suppose that before the existence of man in 
this region much change took place in the enemies against which birds had 
to contend; nor has it ever been suggested that the tastes of female birds 
are as fickle as those of the females of some of the more highly developed 
animals of the Palearctic Region; whilst, on the other hand, there can be 
no doubt that both the food and the migrations of birds must have been 
affected to an enormous extent by the changes of climate consequent on 
the coming on or passing away of glacial epochs. 
Our ignorance of the comparative value of generic characters appears to 
me to be absolute ; and, inasmuch as naturalists have agreed that the name 
of a bird is to be binomial, a combination of the generic and specific names, 
the wisest course is probably to group species together into convenient 
genera, which may assist the memory, taking care to satisfy ourselves that 
the species in each genus are connected together by closer links than those 
which connect them with species in other genera. The lines which Nature 
has drawn between different genera are caused by the extinction of inter- 
mediate species, or by the wideness of the differentiation which has taken 
place between them, which is generally, though not necessarily, a proof of 
the length of time which has elapsed since their original separation. All 
we have to guard against is that the lines which separate our subgenera 
shall be narrower than those which separate the genus from the nearest 
allied genera. 
Our next business is to group our genera into families, the largest of 
which may be conveniently divided into subfamilies. 
So far we shall find it pretty fair sailing in our attempts to classify 
British birds; but when we come to group our families into orders, the 
difference of opinion amongst ornithologists is so great as to the value of 
characters (which date back to such remote ages) as a sign of relationship 
or community of origin, that we are entirely at sea, and can only shrink 
from attempting to decide where doctors disagree. To show the great 
divergence of opinion amongst ornithologists, it is only necessary to com- 
pare the various modern attempts at a scientific classification of birds, 
which will be found to differ from each other in almost every important 
respect, so that it is obvious that any change in the generally received 
classification would be at least premature. Most of these classifications 
are open to the fatal objection that they are attempts to make a linear 
series, beginning with the most highly specialized birds and ending with 
the least so; whereas a true classification must be a chart in which the 
most highly specialized birds are in the centre, and the least so at the cir- 
cumference, where they lead on to the forms most nearly allied to birds. 
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