PRINCIPLES AND PRACTICE OF CLASSIFICATION. 67 
sliifting scenes, degrees of likeness or unlikeness of physical structure indicate with the greatest 
exactitude the nearness or remoteness of organisms in kinship. Morphological characters 
derived from examination of structure are therefore the surest guides we can have to the 
blood-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 branchlet, 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 misinterpreting morphological characters, and so turn- 
ing the key the wrong way. The chief difficulty he will encounter comes 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 morphological 
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 Idbate feet : that is, their feet are 
fitted for swimming purposes 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 different 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. moi-phological, 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 readily as upon 
any other basis: for example, into those that perch, those that walk, those that swim, etc.; 
and, in fact, most early classifications largely rested upon such considerations. It is also evi- 
