INTRODUCTION. ix 
general results are more or less of little value, but the consideration of them 
taken as a whole is of great value when the superficial characters are well 
understood. 
Pterylosis is not well known at present, so that definite conclusions 
cannot be recorded, the only point to emphasise being the continual examina- 
tion of specimens and that criticism of results will later follow. All we now 
Inow is that variation: is seen when not anticipated and consequently 
commonly misunderstood, and the present facts are not of great significance 
through ignorance of basic data. It is a subject that will repay continued 
study, but not occasional excursions. 
Another item continually quoted is the presence or absence of the oil 
gland, and whether it be nude, tufted or feathered. The absurdity of the 
usage of such a character should be obvious. However, it has been seriously 
put forward as of value, but its true insignificance is now generally accepted. 
The presence or absence of the aftershaft and its size are also mentioned, but, 
of course, that character has also been minimised through better material 
and more study, as species which have been commonly cited as showing no 
aftershaft prove to possess one. 
A peculiar feature of the wing feathering was noted, and immediately 
investigated as of import, but here again its absolute value cannot yet be 
appreciated. We refer to the loss of the fifth secondary, though its covert 
is present. This state of the wing has two alternative terms, quincubital 
or eutaxic and aquincubital or diastataxic. The comparative value of this 
state is well seen among the Swifts, where both forms occur. More valuable 
as an absolute character is the coloration of the nestling from the egg onward. 
The state of the nestling when it emerged from the egg was once utilised, 
but this seems of less value than the coloration of the first feathering. That 
this is extremely important is at once seen by study of downy nestlings of 
the Scolopacine group. It is always necessary to remember that secondary 
results are apt to obscure hasty examination in connection with many of the 
above features, but the only conclusion possible from an unprejudiced viewpoint 
is that there are no internal features of outstanding importance, the exceptions 
being so numerous, and the main items so imperfectly investigated ; while 
external characters show less variation and are better known and their develop- 
ment can be more perfectly studied through the much greater mass of material 
available. Consequently throughout this work we have given full details 
of the external features, and also added the internal items as commonly 
given in text books for the benefit of workers—the former all being drawn 
up from actual specimens, and therefore accurate ; the latter compiled from. 
various authoritative sources, and only in few instances, as yet, verified by 
ourselves. 
Before we leave this subject we may enter an apology as to the genus 
splitting complained of in connection with our work. We have given a 
description of the generic characters observed, and have endorsed these with 
a beautiful painting of the character noted. When we have studied all the 
