etc. So there is in this type not only an
equal division of parts on either side, but also a
division above and below, making thus a double symmetry
in the plan, expressed by Baer in the name he gave
it. Baer was perfectly original in his conception
of these four types, for his paper was published in
the very same year with that of Cuvier. But even
in Germany, his native land, his ideas were not fully
appreciated: strange that it should be so,—for,
had his countrymen recognized his genius, they might
have claimed him as the compeer of the great French
naturalist.
Baer also founded the science of Embryology, under
the guidance of his teacher, Dollinger. His researches
in this direction showed him that animals were not
only built on four plans, but that they grew according
to four modes of development. The Vertebrate
arises from the egg differently from the Articulate,—the
Articulate differently from the Mollusk,—the
Mollusk differently from the Radiate. Cuvier only
showed us the four plans as they exist in the adult;
Baer went a step farther, and showed us the four plans
in the process of formation. But his greatest
scientific achievement is perhaps the discovery that
all animals originate in eggs, and that all these
eggs are at first identical in substance and structure.
The wonderful and untiring research condensed into
this simple statement, that all animals arise from
eggs and that all those eggs are identical in the
beginning, may well excite our admiration. This
egg consists of an outer envelope, the vitelline membrane,
containing a fluid more or less dense, the yolk; within
this is a second envelope, the so-called germinative
vesicle, containing a somewhat different and more transparent
fluid, and in the fluid of this second envelope float
one or more so-called germinative specks. At
this stage of their growth all eggs are microsopically
small, yet each one has such tenacity of its individual
principle of life that no egg was ever known to swerve
from the pattern of the parent animal that gave it
birth.
From the time that Linnaeus showed us the necessity
of a scientific system as a framework for the arrangement
of scientific facts in Natural History, the number
of divisions adopted by zooelogists and botanists increased
steadily. Not only were families, orders, and
classes added to genera and species, but these were
further multiplied by subdivisions of the different
groups. But as the number of divisions increased,
they lost in precise meaning, and it became more and
more doubtful how far they were true to Nature.
Moreover, these divisions were not taken in the same
sense by all naturalists: what were called families
by some were called orders by others, while the orders
of some were the classes of others, till it began
to be doubted whether these scientific systems had
any foundation in Nature, or signified anything more
than that it had pleased Linnaeus, for instance, to
call certain groups of animals by one name, while Cuvier
had chosen to call them by another.
