68 
GENERAL ORNITHOLOGY. 
deut, that when functional modifications happen to be coincident with structural affinities, — 
as when the turning of the lower larynx into a music-box coincides with a certain type of 
structure, — such modifications are of the greatest service in classification, as corroborative 
evidence. But since all sound taxonomy rests on morphology, on real structural affinity, we 
must be on our guard against those physiological appearances " which are proverbially 
'^deceptive." I trust I make the principle clear to the student. Its practical application 
is another matter, only to be learned in the school of experience. This matter of 
Homology or Analogy may be thus summed : Birds are homologically related, or 
naturally allied or affined, according to the sum of like structural characters employed for 
similar purposes ; they are analogically related, only according to the sum of unlike characters 
employed for similar purposes. A loon and a cormorant, for instance, are closely affined, 
because they are both fitted in the same way for the pursuit of their prey by flying under water. 
A dipper (family Cinclidce) and a loon (family Colymhidce) are analogous, in so far as both are 
fitted to pursue their prey by flying under water ; but they stand near opposite extremes of the 
ornithological system ; they have little affinity beyond their common birdhood j very different 
structure being modified to attain the same end. So again, conversely, the crow has vocal 
organs almost identical in structure with those of the nightingale, and the organization of the 
two birds is in other respects very similar ; their affinity or homology is therefore close, though 
the crow is a hoarse croaker, the nightingale an impassioned musician. 
The Reason why Morphological Classification is so important as to justify or even 
require its adoption has been very clearly stated by Huxley, whose words I cannot do better 
than quote in this connection. Speaking of animals, not as physiological apparatuses merely ; 
not as related to other forms of life and to climatal conditions ; not as successive tenants of 
the earth ; but as fabrics, each of which is built upon a certain plan, he continues : It is 
possible and conceivable that every animal should have been constructed upon a plan of its 
own, having no resemblance whatever to the plan of any other animal. For any reason we 
can discover to the contrary, that combination of natural forces which we term Life might 
have resulted from, or been manifested by, a series of infinitely diverse structures ; nor would 
anything in the nature of the case lead us to suspect a community of organization between 
animals so different in habit and in appearance as a porpoise and a gazelle, an eagle and a 
crocodile, or a butterfly and a lobster. Had animals been thus independently organized, each 
working out its life by a mechanism peculiar to itself, such a classification as that now under 
contemplation would be obviously impossible ; a morphological or structural classification 
plainly implying morphological or structural resemblances in the things classified. 
" As a matter of fact, however, no such mutual independence of animal forms exists in 
nature. On the contrary, the members of the animal kingdom, from the highest to the lowest, 
are marvellously connected. Every animal has something in common with all its fellows; 
much, with many of them ; more, with a few ; and usually, so much with several, that it 
differs but little from them. 
Now, a morphological classification is a statement of these gradations of likeness which 
are observable in animal structures, and its objects and uses are n:ianifold. In the first place, 
it strives to throw our knowledge of the facts which underlie, and are the cause of, the similar- 
ities discerned, into the fewest possible general propositions, subordinated to one another, 
according to their greater or less degree of generality ; and in this way it answers the purpose 
of a memoria technica, without which the mind would be incompetent to grasp and retain the 
multifarious details of anatomical science. 
But there is a second and even more important aspect of morphological classification. 
Every group in that classification is such in virtue of certain structural characters, which are 
