266 MONTGOMERY. [Vou. XII. 
of allied forms has also been made as nearly equal as possible.” 
The degree of individual variation in regard to the dimensions, 
according to Ridgway, is therefore based (for most of the 
species) on large series of specimens of adult individuals, in 
unworn plumage; and as this distinguished ornithologist’s 
work is regarded as a standard by taxonomists, the accuracy of 
his measurements cannot be questioned. And as such a large 
series of data is the result of years of painstaking work, I may 
be pardoned for not attempting such a labor in the limited time 
at my disposal. I have taken these measurements as given by 
Ridgway, with necessarily the exclusion of such extremes of 
variation as were based upon a very small number of individuals, 
and have computed the percentage of variation for each given 
dimension, expressing the difference between the extremes of 
variation as a percentage of the minor term of variation. In this 
way I have deduced the percentage of variation in the dimensions 
of the larger part of the species and subspecies of North Ameri- 
can birds (together with those of a considerable number of exotic 
species, of casual or possible occurrence within our boundaries) ; 
or, altogether, the species and subspecies of fifty-six families, 
the only omissions being the following small families : 77o- 
gonide, Alcedinide, Momotida, Cotingide, Hirundinide, Am- 
pelide, Laniide, Coerebide, Motacillide, Cinclide, Certhide, 
Syluiide. These latter families have been omitted, because 
their respective scarcity of species would hardly warrant com- 
parisons. Accordingly, using the measurements given by Ridg- 
way as my basis, I have computed the percentage of individual 
variation for one or more of the five dimensions specified, for 
the greater number (approximately 600 or more) of species 
and subspecies of North American birds. It is unnecessary to 
reproduce in this paper these measurements for all the families, 
which would only result in the needless occupation of too much 
valuable space in this JouRNAL ; accordingly, for purposes of 
comparison I will present tables of variation for those families 
only, in which the diagnostic characters of most of the species 
are furnished by the measurements, and for which, therefore, 
the percentages of variation, based upon such necessary accurate 
measurements, may be considered as accurate as possible. 
