Van Beneden, and others, and proceed to give some account of one
investigator who did as much for the progress of Zooelogy as Cuvier, though
he is comparatively little known among us. Karl Ernst von Baer proposed a
classification based, like Cuvier's, upon plan; but he recognized what
Cuvier failed to perceive,--namely, the importance of distinguishing
between type (by which he means exactly what Cuvier means by plan) and
complication of structure,--in other words, between plan and the execution
of the plan. He recognized four types, which correspond exactly to
Cuvier's four plans, though he calls them by different names. Let us
compare them.

_Cuvier_. _Baer_.
Radiates, Peripheric,
Mollusks, Massive,
Articulates, Longitudinal,
Vertebrates. Doubly Symmetrical.

Though perhaps less felicitous, the names of Baer express the same ideas
as those of Cuvier. By the _Peripheric_ he signified those in which all
the parts converge from the periphery or circumference of the animal to
its centre. Cuvier only reverses this definition in his name of
_Radiates_, signifying the animals in which all parts radiate from the
centre to the circumference. By _Massive_, Baer indicated those animals in
which the structure is soft and concentrated, without a very distinct
individualization of parts,--exactly the animals included by Cuvier under
his name of _Mollusks_, or soft-bodied animals. In his selection of the
epithet _Longitudinal_, Baer was less fortunate; for all animals have a
longitudinal diameter, and this word was not, therefore, sufficiently
special. Yet his _Longitudinal_ type answers exactly to Cuvier's
_Articulates_,--animals in which all parts are arranged in a succession of
articulated joints along a longitudinal axis. Cuvier has expressed this
jointed structure in the name _Articulates_; whereas Baer, in his name of
_Longitudinal_, referred only to the arrangement of joints in longitudinal
succession, in a continuous string, as it were, one after another. For the
_Doubly Symmetrical_ type his name is the better of the two; for Cuvier's
name of _Vertebrates_ alludes only to the backbone,--while Baer, who is an
embryologist, signifies in his their mode of growth also. He knew what
Cuvier did not know, that in its first formation the germ of the
Vertebrate divides in two folds: one turning up above the backbone, to
inclose all the sensitive Organs,--the spinal ma