; _second_, the first scientific classification of animals;[6]
_third_, a clear enunciation of the fact of community of plan within
each of the big groups; _fourth_, an attempt to explain certain
instances of the correlation of parts; _fifth_, a pregnant distinction
between homogeneous and heterogeneous parts; _sixth_, a generalisation
on the succession of forms in development; and _seventh_, the first
enunciation of the idea of the _Echelle des etres_.

(1) What surprises the modern reader of the _Historia Animalium_
perhaps more than anything else is the extent and variety of
Aristotle's knowledge of animals. He describes more than 500 kinds.[7]
Not only does he know the ordinary beasts, birds, and fishes with
which everyone is acquainted, but he knows a great deal about
cuttlefish, snails and oysters, about crabs, crawfish (_Palinurus_),
lobsters, shrimps, and hermit crabs, about sea-urchins and starfish,
sea-anemones and sponges, about ascidians (which seem to have puzzled
him not a little!). He has noticed even fish-lice and intestinal
worms, both flat and round. Of the smaller land animals, he knows a
great many insects and their larvae. The extent of his anatomical
knowledge is equally surprising, and much of it is clearly the result
of personal observation. No one can read his account of the internal
anatomy of the chameleon (_Hist. Anim._, ii.), or his description of
the structure of cuttlefish (_Hist. Anim._, iv), or that touch in the
description of the hermit crab (_Hist. Anim._, iv.)--" Two large eyes
... not ... turned on one side like those of crabs, but straight
forward"--without being convinced that Aristotle is speaking of what
he has seen. Naturally he could not make much of the anatomy of small
insects and snails, and, to tell the truth, he does not seem to have
cared greatly about the minutiae of structure. He was too much of a
Greek and an aristocrat to care about laborious detail.

Not only did he lay a foundation for comparative anatomy, but he made
a real start with comparative embryology. Medical men before him had
known many facts about human development; Aristotle seems to have been
the first to study in any detail the development of the chick. He
describes this as it appears to the naked eye, the position of the
embryo on the yolk, the palpitating spot at the third day, the
formation of the body and of the large sightless eyes, the veins on
the yolk, the embryonic membranes, of which he dist