VIII PREFACE. 



and the law of priority, so rigidly applied to genera and species, quite 

 generally ignored. The synonymy of these higher groups, as given, 

 is by no means complete, however, but little time being available for 

 its compilation. 



The matter of determining the limits of families and genera among 

 birds is one of great difficulty, especially among the Passeres; partly 

 because such groups are often not clearly defined, but also because the 

 material necessary for determining such questions is not always avail- 

 able. The question of what constitutes a "family" or a "genus" 

 being involved, and, moreover, one concerning which there is much dif- 

 ference of opinion among systematists, the author's views may be stated, 

 in order to make clear the principles which have been his guide in the 

 present work. 



Accepting evolution as an established fact — and it is difficult to 

 understand how anyone who has studied the subject seriously can by 

 any possibility believe otherwise— there are no "hard and fast lines," 

 no gaps, or "missing links" in the chain of existing animal forms 

 except as they are caused by the extinction of certain intermediate 

 types 5^ therefore, there can be no such group as a family or genus (nor 

 any other for that matter) unless it is cut oflf from other groups by the 

 existence of such a gap; because unless thus isolated it can not be 

 defined, and therefore has no existence in fact. These gaps being very 

 unequally distributed, it necessarily follows that the groups thus formed 

 are very unequal in value; sometimes alternate links in the chain may 

 be missing; again, several in continuous sequence are gone, while occa- 

 sionally a series of several or even numerous links may be intact. It thus 

 happens that some family or generic groups seem very natural or homo- 

 geneous, because the range of generic or specific variation is not great 

 and there is no near approach to the characters of another coordinate 

 group, while others seem verj^ artificial or heterogeneous because 

 among the many generic or specific forms none seem to have dropped 

 out, and therefore, however great the range of variation in structural 

 details, no division into trenchant groups is practicable — not because 

 extreme division would result, but simply because -there can be no 

 proper definition of groups which do not exist.* In short, no group, 

 whether of generic, family, or higher rank, can be valid unless it 

 can be defined by characters which serve to distinguish it from every 

 other. 



In groups of wide geographic range it is of course necessary to have 

 all its components in hand in order to detemiine its limits and the 

 number and boundaries of its subdivisions, for what seem distinct 



'In some cases such apparent gaps are no doubt filled by existing forms which are 

 as yet unknown to science. 

 ^ Certain so-called genera of the family Fringillida) afford a sirikiug illustration. 



