70 
GENEBAL OBNITHOLOGY. 
for the tarsus of such a bird is differently disposed, though a lark has an elaborate singing 
apparatus, and only nine instead of ten developed primaries. Once more :' the development 
of a keeled sternum, a peculiar saddle-shape of certain vertebrae, and lack of true teeth, are 
characters coexisting in all the higher birds ; and, as far as these birds are concerned, we 
have no hint that such a combination is ever broken. In fact, however, the singular Creta- 
ceous Ichthyornis shows us a pattern of bird in which a well-keeled sternum and perfectly 
formed wing coexist with teeth in reptile-like jaws and with fish-like biconcave vertebrae. 
What we learn from this case indeed breaks down one of the most precise definitions we 
might have made (and indeed did make) respecting birds at large ; but in its failure we are 
taught how great is the modification of geologically recent birds from their primitive gener- 
alized ancestry; we learn something likewise of the steps of such modification, and of the 
length of time required for the process. It is the history of attempts to frame definitions 
of groups in zoology, that they are all liable to be negatived by new discoveries, and 
therefore to be broken down and require remodelling as our knowledge increases. It is to 
be readily perceived that the ability to draw distinctions and make definitions of groups is as 
much the gauge of our ignorance as the test of our knowledge ; for all groups, like all species, 
come to be such by modification so gradual, so slight in each successive increment of difference, 
that, if all the steps of the process were before our eyes, we should be able to limit no groups 
whatever in a positive, unqualified manner. All would merge insensibly into one another, be 
inseparably linked in as many series as there have been actual lines of evolutionary progress, 
and finally converge to the one or few starting points of organized beings. 
Practically, however, the case is quite the reverse, — happily for the comfort of the work- 
ing naturalist, however sadly the philosopher may deplore the ignorance implied. Degrees of 
likeness and unlikeness do exist, which when rightly interpreted enable us to mark off groups 
of all grades with much facility and precision, and thus erect a morphological classification 
which recognizes and defines such degrees, and explains them upon the principles of Evolution. 
The way in which the principles of such classification are to be practically applied gives occa- 
sion for some further remarks upon 
Zoological Characters. — A character," in zoological language, is any point of struc- 
ture which may be perceived and described for the purpose of comparing or contrasting animals 
with one another. Thus, the conditions of the sternum, palate, tarsus, larynx, as noted in 
preceding paragraphs, are each of them " characters " which may be used in describing indi- 
vidual birds, or in framing definitions of groups of birds. Morphological characters, with 
which the classification we have adopted alone concerns itself, may be derived from the 
structure of a bird considered in any of its relations, or as affected by any of the conditicms to 
which it is subjected. Thus emhryological characters are those afforded by the bird during 
the progress of its development in the egg, from the almost structureless germ to the fully 
formed chick. Such characters of the embryo in its successive stages are of the utmost signifi- 
cance ; for it is a fact, that the germ of each of the higher organisms goes through a series of 
developmental changes w'hich, at each succeeding step in the unfolding of its appropriate plan 
of structure, causes it to resemble the adult state of animals lower than itself in the scale of 
organization. In fine, the history of the evolution of every individual bird epitomizes the 
history of those changes which birds collectively have undergone in becoming what they are by 
modified descent from lower organisms. Such transitory stages of any embryo, therefore, give 
us glimpses of those evolutionary processes which have affected the group to which it belongs. 
Any bird, for example, when a germ, is at first on the plane of organization of the very lowest 
known creatures, — one of the Protozoa. As its germ develops, and its structure becomes 
more complicated by the formation of parts and organs successively differentiated and special- 
ized, it rises higher and higher in the scale of being. At a certain stage very early reached 
