SEC. II rKINCIPLES AND PRACTICE OF CLASSIFICATION 109 



in taxonomy ; being of nil grades, from the trivial ones that serve 

 to distinguish the nearest related species or varieties, to the 

 fundamental ones that serve to mai'k off primary divisions. Thus 

 the " character " of possessing a backbone is common to all animals 

 of an immense series called Vertebraia. 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 Carinatce ; of feet 

 fitted for perching to all the order Passeres ; of a musical apparatus 

 to all the sub-order Oscines / of nine primaries to all the family 

 Fringillidcc ; 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 

 imj)ressions. It will also be obvious that every iilterior modifica- 

 tion 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 resvdts 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 common ancestry ; 

 and that, being dependent upon that process, they are capable of 

 explaining it if rightly interpreted. For example : Two white- 

 Avinged crossbills, hatched in the same nest, scarcely differ percep- 

 tibly (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 character." All 

 the individual crossbills which exhibit this particular sum consti- 

 tute a " species." In this case, the genetic relationship of offspring 

 and parent is unquestionable — it is an observed fact. Now turn 

 to the extremely opposite case. The difference between our cross- 

 bills and the Cretaceous Ichthyornis is enormous : I suppose it is 

 nearly the greatest known to subsist between any two birds what- 

 soever. But the Ichfhi/ornis and the Loxia are also separated by 

 a correspondingly immense interval of time, and presumably by 

 correspondingly enormous differences in conditions of environment — 

 in their physical surroundings. It is a logical inference that these 

 two things — difference in physical structure and difference in 



