PRINCIPLES AND PRACTICE OF CLASSIFICATION. 67 



pedigrees and constructing the pMjlum; it is like tracing any leaf or twig of a tree to its 

 branclilet, this to its bough, this to its trunk or main stem. The student will readily perceive, 

 from what has been said, the impossibility of naturally arranging any considerable number of 

 birds in any linear series of groups, one after the other. To do so is a mechanical necessity of 

 book-making, where groups have to succeed one another, on page after page. Some groups 

 will follow naturally ; others will not ; no connected chain is possible, because no such single 

 continuous series exists in nature. In cataloguing, or otherwise arranging a series of birds for 

 description, we simply begin with the highest groups, and make our juxtapositions as well as 

 we can, in order to have the fewest breaks in the series. 



Morphology being the only safe clue to natural affinities, and the key to all rational 

 classification, the student cannot too carefully consider what is meant by this term, or too 

 sedulously guard against misinterpreting morphological characters, and so turning the key the 

 wrong way. The chief difficulty he will encounter comes from physiological adaptations of 

 structure ; and this is something that must be understood. The expression means that birds, 

 or any animals, widely different in their morphological characters, may have certain parts of 

 their organization modified in the same way, thus bringing about a seemingly close resem- 

 blance between organisms not nearly related to each other. For example : a Phalarope, a 

 Coot, and a Grebe, all have lobate feet — that is, their feet are fitted for swimming in the same 

 way, namely, by development of flaps or lobes on the toes. A striking but superficial and 

 therefore unimportant resemblance iu a certain particular exists between these birds, on the 

 strength of which they used to be classed together in a group called Pinnatipedes, or " fin- 

 footed " birds. But, on sufficient examination, these three birds are found to be very unlike 

 in other respects; the sum of their unlikenesses requires us to separate them quite widely in 

 any natural system. The group Pinnatipedes is therefore unnatural, and the appearance of 

 affinity is proven to be deceptive. Such resemblance in the condition of the feet is simply 

 functional, or physiological, and is not correspondent with structural or morphological relation- 

 ships. The relation between these three birds is analogical ; it is an inexact superficial resem- 

 blance between things profoundly unlike, and therefore having little homological or exact 

 relationship. Analogy is the appai-ent resemblance between things really unlike — as the 

 wing of a bird and the wing of a bntterfiy, as the lungs of a bird and the gills of a fish. 

 Homology is the real resemblance or true relation between things, however different they may 

 appear to be — as the wing of a bird and the foreleg of a horse, the lungs of a bird and the 

 swim-bladder of a fish. Analogy commonly rests upon mere functional, i. e. physiological, 

 modifications ; homology is grounded upon structural, i. e. morphological, identity or unity. 

 Analogy is the correlative of physiology, homology of morphology; but the two may be coin- 

 cident, as when identical structures are used for the same purposes and are therefore physio- 

 logically identical. Physiological diversity of structure is incessant, and continually interferes 

 with morphological identity of structure, to obscure or obliterate the indications of affinity the 

 latter would otherwise express clearly. It is obvious that birds might be classified physiologi- 

 cally, according to their adaptive modifications or analogical resemblances, just as readily as 

 upon any otlier basis : for example, into those that perch, those that walk, those that swim, 

 etc. ; in fact, most early classifications rested upon such considerations. It is also evident, 

 that when functional modifications happen to be coincident with structural affinities — as when 

 the turning of the lower larynx into a music-box coincides with a certain type of structure — 

 such modifications are of the greatest possible service in classification. But since all sound 

 taxonomy rests on morphology, on real structural affinity, we must be on our guard against 

 those physiological "appearances" which are proverbially "deceptive." I trust I make the 

 principle clear to the student. Its practical application is another matter, only to be learned 

 in the school of experience. This question of 



