s
essentially a progress from the general to the special (p. 242).
Botanists have not been troubled by any recapitulation theory, and in
founding their big groups, Acotyledons, Monocotyledons, and
Dicotyledons, upon embryological characters, they were guided by true
principles, which ought indeed to be followed in zoology. If we knew the
development of all kinds of animals sufficiently well, then the best way
to classify them would be according to the characters they show in their
early development, for it is in early development that they show the
characters of the type in their most generalised form. As it is, we have
in our ignorance to establish the big groups by the study of adult
structure, but we find, on putting together all we know of comparative
embryology, that a classification of animals according to the mode of
their development gives, as is only natural, the same four groups as
does the study of adult structure. The four types of development are
thus:--

(1) The double-symmetrical, which is found in Vertebrates. It is called
the double-symmetrical, because in Vertebrates development takes place
from a central axis (notochord) in two directions, upwards and
downwards, in such a way that two tubes are formed, one above and one
below the axis. (2) The second type is the symmetrical, which is shown
by Annulates. A primitive streak is formed on the ventral surface of the
yolk; development proceeds symmetrically on both sides of the streak.
(3) Radiate development is probably typical of the radiate structural
type. (4) In the massive type, the development seems to be a spiral one.

Common to most modes is a separation of the germ into animal and plastic
layers, a separation which seems to be conditioned largely by the
presence of yolk. A classification based upon embryological characters
ought to be applied even to the lesser groups and would here prove
itself of service. Embryology, for instance, fully supports de
Blainville's separation of Batrachia from true reptiles,[176] for reptiles
develop an amnion and Batrachia do not.

We come now to the sixth and last Scholion. Development is a true
evolution of the special from the general, so runs von Baer's most
general law of all. This can be expressed in a slightly different way,
and the words which he chooses in the sixth Scholion to express this
final and most general result are these:--"The developmental history of
the individual is the history of the growi