Form and Function | Page 5

"genera" he distinguishes. As regards Testacea, he writes, "The
nature of their internal structure is similar in all, especially in the
turbinated animals, for they differ in size and in the relations of excess;
the univalves and bivalves do not exhibit many differences" (Cresswell,
loc. cit., p. 83). There is an interesting remark about "the creature called
carcinium" (hermit-crab), that it "resembles both the Malacostraca and
the Testacea, for this in its nature is similar to the animals that are like
carabi, and it is born naked" (Cresswell, loc. cit., p. 85). In the last
phrase we may perhaps read the first recognition of the embryological
criterion.
With the recognition of unity of plan within each group necessarily
goes the recognition of what later morphology calls the homology of
parts. The parts of a horse can be compared one by one with the parts
of another viviparous quadruped; in all the animals belonging to the
same class the parts are the same, only they differ in excess or

defect--these remarks are placed in the forefront of the Historia
Animalium. Generally speaking, parts which bear the same name are
for Aristotle homologous throughout the class. But he goes further and
notes the essential resemblance underlying the differences of certain
parts. He classes together nails and claws, the spines of the hedgehog,
and hair, as being homologous structures. He says that teeth are allied
to bones, whereas horns are more nearly allied to skin (Hist. Anim., iii.).
This is an astonishingly happy guess, considering that all he had to go
upon was the observation that in black animals the horns are black but
the teeth white. One cannot but admire the way in which Aristotle fixes
upon apparently trivial and commonplace facts, and draws from them
far-reaching consequences. He often goes wrong, it is true, but he
always errs in the grand manner.
While Aristotle certainly recognised the existence of homologies, and
even had a feeling for them, he did not clearly distinguish homology
from analogy. He comes pretty near the distinction in the following
passage. After explaining that in animals belonging to the same class
the parts are the same, differing only in excess or defect, he says, "But
some animals agree with each other in their parts neither in form nor in
excess and defect, but have only an analogous likeness, such as a bone
bears to a spine, a nail to a hoof, a hand to a crab's claw, the scale of a
fish to the feather of a bird, for that which is a feather in the bird is a
scale in the fish" (Cresswell, loc. cit., p. 2). One of these comparisons is,
however, a homology not an analogy, and the last phrase throws a little
doubt upon the whole question, for it is not made clear whether it is
position or function that determines what are equivalent organs.
In the De Partibus Animalium there occurs the following
passage:--"Groups that only differ in degree, and in the more or less of
an 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."[8] It is thus similarity in form
and structure which determines the formation of the main groups.
Within each group the parts differ only in degree, in largeness or
smallness, softness and hardness, smoothness or roughness, and the like
(loc. cit., i., 4, 644^b). These passages show that Aristotle had some
conception of homology as distinct from analogy. He did not, however,
develop the idea. What Aristotle sought in the variety of animal
structure, and what he found, were not homologies, but rather
communities of function, parts with the same attributes. His interest
was all in organs, in functioning parts, not in the mere spatial
relationship of parts.
This comes out clearly in his treatise On the Parts of Animals, which is
subsequent to, and the complement of, his History of Animals. The
latter is a description of the variety of animal form, the former is a
treatise on the functions of the parts. He describes the plan of the De
Partibus Animalium as follows:--"We have, then, first to describe the
common functions, common, that is, to the whole animal kingdom, or
to certain large groups, or to members of a species. In other words, we
have to describe the attributes common to