THE RING-TAILED AND GOLDEN EAGLES. 437 
Besides the strongly-marked distinction in the plumage 
of the tail, the colour of the iris is usually different in these 
birds, that of the Golden Eagle being yellow, that of the 
Ring-tail hazel. This character, I am aware, is subject to 
variation ; and I therefore mention it under the objection, 
arising from that uncertainty, which many may annex to 
its value as a specific character. But it may fairly be ar- 
gued, that every external character, taken singly, is subject 
to exhibit occasional discrepancies, and that this character 
is as permanent, and therefore as valuable, as many others ; 
not, to be sure, as a positive specific distinction viewed 
singly, but yet as a very strong corroboration, when con- 
sidered in its generality, and as connected with other cir- 
cumstances. It may be v/orth while, moreover, to consider 
for a moment the nature, as well as the value, of this varia- 
tion. There is no doubt, in the first place, of the fact, that 
the colour of the iris in these birds is usually different, the 
Golden Eagle's being, as I have stated, bright yellow, the 
Ring-taiFs deep hazel. Now, if it can be shewn, that in 
either of these, or in any other species, there is a regular 
and customary change of the iris from one colour to an- 
other taking place, like the changes in the colouring of the 
plumage, in consequence of an advance to maturity, then 
I confess that the argument to be derived from a disagree- 
ment in this character between the two birds in question 
would be greatly, if not altogether, invalidated. But I 
maintain, that this imagined change does not take place, 
or at least has never yet been ascertained, or even asserted 
to take place, in consequence of an advance from youth to 
age ; that it is an accidental circumstance taking place 
rarely, and irregularly, and not characteristic of, or in any 
way connected with, a particular period of life. It follows, 
therefore, that this distinction, being usual between the 
birdy hitherto named the Golden and the Ring-tailed Eagles, 
