No. I.] ORGANIC VARIATION. 265 
have found domestication to cause a more or less continuous 
development. 
In order to test the correctness of the assumption that indi- 
vidual variation is most marked (1) in those species which pos- 
sess geographical races, and (2) in those species which undertake 
extensive migrations, I have examined nearly all the species of 
North American birds with reference to individual variation in 
some or all of the following dimensions: culmen of the bill, 
wing (from carpal joint to tip of longest primary), tarsus (so- 
called, but really tarso-metatarsus), whole length (from tip of 
bill to tip of tail), and tail (from the pygostyle to tip of the 
longest rectrix). It was my original intention to personally 
undertake all the measurements, and with that object in view 
I commenced a series of detailed measurements upon the bird- 
skins in the collection of the Academy of Natural Sciences of 
Philadelphia. Unfortunately for me, however, this collection 
did not offer large enough series of individuals of all the species 
desired, and not having the opportunity nor time to study other 
large collections, —namely those at Cambridge, New York, 
and Washington, —I was obliged to desist from further personal 
examinations. In lieu, then, of such direct examination, I have 
taken Robert Ridgway’s excellent “ Manual of North American 
Birds, 1887 ” (first ed.) as my authority for the extremes of indi- 
vidual variation, in regard to the dimensions specified, of the North 
American species of birds. And here I would express my 
hearty gratitude to Professor Ridgway for his liberality and 
generosity in allowing me to make use of his valuable data. In 
speaking of the measurements given in his work, Ridgway 
states (p. ix): “Whenever practicable, they have been taken 
from large series of specimens, and the extremes given as well 
as the average. . . . In the case of closely allied forms, or 
where distinctive characters are largely a matter of dimensions 
or the proportionate measurements of different parts, care has 
been taken to measure, whenever possible, an equal number of 
specimens of the several forms to be compared; and specimens 
in abraded or otherwise imperfect plumage, as well as young 
birds, have been excluded. When there is any marked sexual 
difference in size, the number of males and females measured 
