PRINCIPLES AND PBAQTICE OF CLASSIFICATION. 71 



(for the steps by whicli it becomes like any mvertebrate are very speedily passed over), it 

 resembles a fish in possessing gill-like sUts, several aortic arches, no true kidneys, no amnion, 

 etc. Further advanced, losing its giUs, gaining kidneys and amnion, etc., it rises to the 

 dignity of a reptile, and at this stage it is more like a reptile than like a bird ; having, for 

 example, a number of separate bones of the wrist and ankle, no feathers, etc. The assump- 

 tion of its own appropriate characters, i. e. those by which it passes from a reptilian creature 

 to become a bird, is always the last stage reached. We can thus actually see and note, 

 inside any egg-sheU, exactly those progressive steps of development of the individual bird 

 which we believe to have been taken on a grand scale in nature for the evolution of the class 

 Aves from lower forms of life ; and the lesson learned is fraught with significance. It is nothing 

 less than the demonstration in ontogeny (genesis of the individual) of that phylogeny by which 

 groups of creatures come to be. The interior of any adult bird, again, furnishes us with all 

 kinds of ordinary anatomical characters, derived from the way we perceive the different organs 

 and systems of organs to be fashioned in themselves, and arranged with reference to one 

 another. The finishing of the outward parts of a bird gives us the ordinary external characters, 

 in the way in which the skin and its appendages are modified to form the covering of the bUl 

 and feet, and to fashion all kinds of feathers. Birds being of opposite sexes, and such difler- 

 enoe being not only indicated in tlie essential sexual organs, but usually also in modifications 

 in size or shape of the body or quality of the plumage and other outgrowths, a set of sexual 

 characters are at our service. Birds are also sensibly modified in their outward details of 

 feathering by times of the year when the plumage is changed, and this renders appreciation 

 of seasonal characters possible. All such circumstances, and others that could be mentioned, 

 such as effects of climate, of domestication, etc., in so far as they in any way affect the struc- 

 ture of birds, conspire to produce zoological "characters,'' as these are above defined. Such 

 characters, according as they result from more or less profound impressions made upon the 

 organism, are of more or less "value " in taxonomy ; being of all grades, from the trivial ones 

 that serve to distinguish the nearest related species or varieties, to the fundamental ones that 

 serve to mark off primary divisions. Thus the "character" of possessing a backbone is com- 

 mon to all animals of an immense series, called Vertebrata. The "character" of feathers is 

 common to all the class Aves ; of toothless jaws to all modem birds ; of a keeled sternum to 

 all the sub-class Carinatcs ; of feet fitted for perching to all Passeres ; of a musical apparatus 

 to all Oscmes ; of nine primaries to aU FringilKdce ; of crossed mandibles to all of the genus 

 Loxia ; of white bands on the wings to all of the species Loxia leucoptera. There is thns 

 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 

 impressions. It will also be obvious, that every ulterior modification presupposes inclusion 

 of all the prior ones ; for a whitcwinged crossbill, to be itself, must be a loxian, fringilline, 

 oscine, passerine, carinate, modern, avian, vertebrated animal. The more characters, of aU 

 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 results in 



Degrees of Iiikeness.— Were all birds alike, or did they all difter 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 -Sdthin the limits of their Avian nature ; it is a matter 

 of belief, that these degrees are the necessary result of Evolution,— of descent Vidth 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-winged crossbills, hatched in 

 the same nest, scarcely differ perceptibly (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 



