SEC. 11 PRINCIPLES AND PRACTICE OF CLASSIFICATION 107 



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 

 organised beings. 



Practically, however, the case is quite the reverse, — happily for 

 the comfort of the working 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 recognises 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 occasion for some further remarks upon 



Zoological Characters. — A " character," in zoological language, 

 is any point of structure 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 individual 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 

 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 

 significance ; 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 organisation. In fine, the history of the 

 evolution of every individual bird epitomises the history of those 

 changes which birds collectively have undergone in becoming what 



