re
and origin of the principal veins are the same, but the multitude of
smaller veins is not alike in all, for neither are the parts of the
same nature, nor do all possess the same parts" (Cresswell, _loc.
cit._, p. 56). It will be noticed in the first and last of these three
quotations that Aristotle recognises the fact of correlation between
systems of organs,--between limbs and bones, and between blood-vessels
and the parts to which they go.

Sanguineous animals all possess certain organs--heart, liver, spleen,
kidneys, and so on. Other organs occur in most of the classes--the
oesophagus and the lungs. "The position which these parts occupy is
the same in all animals [sc. Sanguinea]" (Cresswell, _loc. cit._, p.
39).

Unity of plan is observable not only in the Sanguinea, but also within
each of the other large groups. Aristotle recognises that all his
cuttlefish are alike in structure. Among his Malacostraca he compares
point by point the external parts of the carabus (_Palinurus_), and
the astacus (_Homarus_), and he compares also the general internal
anatomy of the various "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