PRINCIPLES AND PRACTICE OF CLASSIFICATION. 69 



bones, are fouud in combination, because all these modilicatious of structure are evidently re- 

 lated to loss of power of flight ; and, in fact, no exception is known to the generalization, that 

 such conditions of sternal, coraco-scapular, and humeral bones always coexist. But in all 

 known struthious (ratite) birds, this state of the bones in mention coexists with a peculiar 

 modification of bones of the palate, and no necessary connection between these two sets of 

 diverse characters is conceivable. Now, if we only knew struthious birds, and found the com- 

 bination in mention to hold with them all, we should doubtless declare our belief, that any 

 bird having such palatal characters would also be found to possess such imperfect wing-appa- 

 ratus. But this would be going too far ; for we know that Tinamous (Dromoeognathce) have 

 such a palate, yet have a keeled sternum and functionally developed wings. To take another 

 case, derived from consideration of a large number of existing birds: it is an observed fact, that 

 a particular arrangement of plates upon the back of the tarsus, a peculiar modification of the 

 lower larynx or voice organ, and an undeveloped or abortive condition of the first large feather 

 on the hand, are found associated in a vast series of birds, constituting the group of Passeres 

 called Oscines. What possible connection there can be between these three separate and ap- 

 parently independent modifications we cannot even surmise ; but that they have some natural 

 and necessary connection we cannot doubt, and that the connection is causal, not fortuitous, is 

 a logical inference from the observed fact, that birds which present this particular combination 

 are also closely related in other structural characters — that is, that they have all been sub- 

 jected to operative influences which have conspired to produce the modifications observed. 

 Given, then, a bird with a known oscine larynx, but unknown as to its feet and wings, it 

 would be a reasonable inference that these members, when discovered, would present the char- 

 acters observed to occur in like cases. But the first Lark (Alaudidce) examined would show 

 this inference to be fallible ; for the tarsus of such a bird is diSerently 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 vertebrfe, 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 Cretaceous 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 

 vertebras. 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 there- 

 fore 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, uuqualified 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 ofl' 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 



