PRINCIPLES AND PRACTICE OF CLASSIFICATION. 71 
(for the steps by which it becomes like any invertebrate are very speedily passed over), it 
resembles a fish in possessing gill-like slits, several aortic arches, no true kidneys, no amnion, 
etc. Further advanced, losing its gills, gaining kidneys and amnion, etc., it rises to the 
dignity of a reptile, and at this stage it is more like a reptile than like a bird; having, for 
example, a number of separate bones of the wrist and ankle, no feathers, etc. The assump- 
tion of its own appropriate characters, 7. e. those by which it passes from a reptilian creature 
to become a bird, is always the last stage reached. We can thus actually see and note, 
inside any egg-shell, exactly those progressive steps of development of the individual bird 
which we believe to have been taken on a grand scale in nature for the evolution of the class 
Aves from lower forms of life; and the lesson learned is fraught with significance. It is nothing 
less than the demonstration in ontogeny (genesis of the individual) of that phylogeny by which 
groups of creatures come to be. The interior of any adult bird, again, furnishes us with all 
kinds of ordinary anatomical characters, derived from the way we perceive the different organs 
and systems of organs to be fashioned in.themselves, and arranged with reference to one 
another. The finishing of the outward parts of a bird gives us the ordinary external characters, 
in the way in which the skin and its appendages are modified to form the covering of the bill 
and feet, and to fashion all kinds of feathers. Birds being of opposite sexes, and such differ- 
ence being not only indicated in the essential sexual organs, but usually also in modifications 
in size or shape of the body or quality of the plumage and other outgrowths, a set of sexual 
characters are at our service. Birds are also sensibly modified in their outward details of 
feathering by times of the year when the plumage is changed, and this renders appreciation 
of seasonal characters possible. All such circumstances, and others that could be mentioned, 
such as effects of climate, of domestication, etc., in so far as they in any way affect the struc- 
ture of birds, conspire to produce zodlogical ‘‘ characters,” as these are above defined. Such 
characters, according as they result from more or less profound impressions made upon the 
organism, are of more or less “‘ value” in taxonomy ; being of all grades, from the trivial ones 
that serve to distinguish the nearest related species or varieties, to the fundamental ones that 
serve to mark off primary divisions. Thus the ‘‘ character” of possessing a backbone is com- 
mon to.all animals of an immense series, called Vertebrata. The “character” of feathers is 
common to all the class Aves ; of toothless jaws to all modern birds; of a keeled sternum to 
all the sub-class Carinate ; of feet fitted for perching to all Passeres ; of a musical apparatus 
to all Oscines ; of nine primaries to all Fringillide ; of crossed mandibles to all of the genus 
Loxia ; of white bands on the wings to all of the species Loxia leucoptera. There is thus 
seen a sliding scale of valuation of characters, from those involving the most profound or 
primitive modifications of structure to those resting upon the most superficial or «ultimate 
impressions.. It will also be obvious, that every ulterior modification presupposes inclusion 
of all the prior ones ; for a white-winged crossbill, to be itself, must be a loxian, fringilline, 
oscine, passerine, carinate, modern, avian, vertebrated animal. The more characters, of all 
grades, that any birds share in common, the more closely are they related, and conversely. 
Obviously, the possession of more or fewer characters in common results in 
Degrees of Likeness. — Were all birds alike, or did they all differ by the same characters 
to the same degree, no classification would be possible. It is a matter of fact, that they do 
exhibit all degrees of likeness possible within the limits of their Avian nature ; it is a matter 
of belief, that these degrees are the necessary result of Evolution,— of descent with modification 
from a comimon ancestry; and that being dependent upon that process, they are capable of 
explaining it if rightly interpreted. For example: Two white-winged crossbills, hatched in 
the same nest, scarcely differ perceptibly (except in sexual characters) from each other and 
from the pair that laid the eggs. We call them “specifically” identical ; and the sum of the 
differences by which they are distinguished from any other kinds of crossbills is their “specific 
