On The Parts Of Animals   

identical element that they possess, are aggregated under a single
class; groups whose attributes are not identical but analogous are
separated. For instance, bird differs from bird by gradation, or by
excess and defect; some birds have long feathers, others short ones,
but all are feathered. Bird and Fish are more remote and only agree in
having analogous organs; for what in the bird is feather, in the fish
is scale. Such analogies can scarcely, however, serve universally as
indications for the formation of groups, for almost all animals
present analogies in their corresponding parts.
The individuals comprised within a species, such as Socrates and
Coriscus, are the real existences; but inasmuch as these individuals
possess one common specific form, it will suffice to state the
universal attributes of the species, that is, the attributes common to
all its individuals, once for all, as otherwise there will be endless
reiteration, as has already been pointed out.
But as regards the larger groups-such as Birds-which comprehend many
species, there may be a question. For on the one hand it may be urged
that as the ultimate species represent the real existences, it will be
well, if practicable, to examine these ultimate species separately,
just as we examine the species Man separately; to examine, that is,
not the whole class Birds collectively, but the Ostrich, the Crane,
and the other indivisible groups or species belonging to the class.
On the other hand, however, this course would involve repeated mention
of the same attribute, as the same attribute is common to many
species, and so far would be somewhat irrational and tedious. Perhaps,
then, it will be best to treat generically the universal attributes of
the groups that have a common nature and contain closely allied
subordinate forms, whether they are groups recognized by a true
instinct of mankind, such as Birds and Fishes, or groups not popularly
known by a common appellation, but withal composed of closely allied
subordinate groups; and only to deal individually with the attributes
of a single species, when such species, man, for instance, and any
other such, if such there be-stands apart from others, and does not
constitute with them a larger natural group.
It is generally similarity in the shape of particular organs, or of
the whole body, that has determined the formation of the larger
groups. It is in virtue of such a similarity that Birds, Fishes,
Cephalopoda, and Testacea have been made to form each a separate
class. For within the limits of each such class, the parts do not
differ in that they have no nearer resemblance than that of
analogy-such as exists between the bone of man and the spine of
fish-but differ merely in respect of such corporeal conditions as
largeness smallness, softness hardness, smoothness roughness, and
other similar oppositions, or, in one word, in respect of degree.
We have now touched upon the canons for criticizing the method of
natural science, and have considered what is the most systematic and
easy course of investigation; we have also dealt with division, and
the mode of conducting it so as best to attain the ends of science,
and have shown why dichotomy is either impracticable or inefficacious
for its professed purposes.
Having laid this foundation, let us pass on to our next topic.
Part 5
Of things constituted by nature some are ungenerated, imperishable,
and eternal, while others are subject to generation and decay. The
former are excellent beyond compare and divine, but less accessible to
knowledge. The evidence that might throw light on them, and on the
problems which we long to solve respecting them, is furnished but
scantily by sensation; whereas respecting perishable plants and
animals we have abundant information, living as we do in their midst,
and ample data may be collected concerning all their various kinds, if
only we are willing to take sufficient pains. Both departments,
however, have their special charm. The scanty conceptions to which we
can attain of celestial things give us, from their excellence, more
pleasure than all our knowledge of the world in which we live; just as