PRINCIPLES AND PRACTICE OF CLASSIFICATION. 67 



If liifting scenes, degrees of likeness or unlikeness of physical structure indicate with the greatest 

 exactitude the nearness or remoteness of organisms in kinsliip. Morphological characters 

 derived from examination of structure are therefore the surest guides we can have to the 

 hlood-relationships we desire to establish; and such relationships are the "natural affinities" 

 which all classification aims to discover and formulate. As already said, taxonomy consists 

 in tracing pedigrees, and constructing the phylum ; it is like tracing any leaf or twig of a tree 

 to its hranclilet, this to its bough, this again to its trunk or main stem. The student will 

 readily perceive, from what has been said, the impossibility of naturally arranging any consid- 

 erable number of birds in any linear series of groups, one after the other. To do so means 

 nothing more or less than the mechanical necessity of book-making, where groups have to 

 succeed one another, in writing 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 juxta-positions as well as we can, in order 

 to have the fewest breaks in the series. 



Morphology being the safest, indeed 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 misiuterjireting morphological characters, and so turn- 

 ing the key the wrong way. The chief difficulty he will encounter conies from physiological 

 adaptations of structure ; and this is something that must be thoroughly understood. The 

 expression means that birds, or any animals, widely difi"erent in the sum of their moi-phological 

 characters, may have certain parts of their organization modified in the same way, thus bring- 

 ing about a seemingly close resemblance between organisms really little 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 purjioses in the. same way, namely, by development of flaps or lobes on 

 the toes. A striking but very superficial and therefore unimportant resemblance in 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 resem- 

 blance in the condition of the feet is simply functional, or physiological, and is not correspon- 

 dent with structural or morphological relationships. The relation, in short, between these 

 three birds is analogical ; it is an inexact superficial resemblance between things profoundly 

 unlike, and therefore having little homological or exact relationship. Analogy is the apparent 

 resemblance between things really unlike, — as the wing of a bird and the wing of a butterfly, 

 as the lungs of a bird and the gills of a fish. Homology is the real resemblance, or true relation 

 between things, however difierent they may appear to be, — as the wing of a bird and the fore- 

 leg of a horse, the lungs of a bird and the swim-bladder of a fish. The former commonly 

 rests upon mere functional, i. e. physiological, modifications ; the latter is grounded upon 

 structural, i. e. morphological, identity or unity. Analogy is the correlative of physiology, 

 homology of morphology ; but the two may be coincident, as when structures identical in 

 morphology are used for the same purposes and are therefore physiologically identical. Physi- 

 ological 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 physiologically, 

 according to their adaptive modifications or analogical resemblances, just as I'eadily as upon 

 any other basis: for example, into those that perch, those that walk, those that swim, etc. j 

 and, in fact, most early classifications largely rested upon such considerations. It is also evi- 



