70 GENERAL ORNITHOLOGY. 



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 stenium, 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 tact, however, the singular Creta- 

 ceous Ichthyornis shows us a pattern of bird in which a well- keeled sternum and perfectly 

 formed wiag 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 yve 

 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 ; Ave learn something likewise of tlie 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 aU species, 

 come to be such by modification so gradual, so slight in each successive increment of difference, 

 that, if all tlie 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 uulikeness do exist, which when rightly interpreted enable us to mark off groups 

 of all grades with much faciUty 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 conditions to 

 which it is subjected. Thus embryological characters are those afforded by the bfrd 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 which, 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 



