84 INTRODUCTION. 
inconstant character, and in but few cases can it be de- 
pended upon. This has been used as a distinguishing 
mark in separating some of the smaller Flycatchers. I 
have tested it, and found it to be valicless, as there is no 
rule relative to age or sex by which this is governed. 
Again, intensity and paleness of color have been almost 
unanimously considered of specific value. Indeed, some 
species have been formed wholly upon this peculiarity ! 
(Witness Zurdus Alice.) Spots and bars on the wings, 
and streaks on the ramp, are characters changing with 
age and season, and should xever be depended upon. 
Another th'ng is the difference in size; any one who 
has collected, and carefully measured, birds of one species 
from one locality, in any numbers, will at once be con- 
vinced of the absurdity of paying any attention to this 
particular in determining specific characters. As perti- 
nent, I wish here to announce a somewhat surprising dis- 
covery that I have made. After a careful measurement 
of over three thousand specimens, I have been convinced 
of the fact, that birds for a certain period increase in size, 
after which they gradually decrease. Whether the period of 
decrease is limited or not during the life of the bird, I am 
unable at present to state. The period of zncrease may also 
-be variable, both specifically and individually, which yet re- 
mains to be proven. Both the increase and decrease are 
proportional ; feet, bill, wings, and body alike keep equal 
pace. This rule is not without its exceptions, but in the 
majority of cases it is the rule, and I offer it to my fellow- 
laborers to prove and use in their ornithological investi- 
gations. 
Besides those named, there are other distinctions used 
in determining species, that in some cases are inconstant. 
I think it advisable always, before attempting to sepa- 
rate a supposed species from one closely allied, to procure 
a sufficiently large number of specimens, and carefully 
