oint, according to the lower or higher goal it has to
reach.

One important consequence for morphology results from von Baer's laws of
differentiation within the type. If the embryo develops from the general
to the special, then the state in which each organ or organ-system first
appears must represent the general or typical state of that organ within
the group. Embryology will therefore be of great assistance to
comparative anatomy, whose chief aim it is to discover the generalised
type, the common plan of structure, upon which the animals of each big
group are built. And the surest way to determine the true homologies of
parts will be to study their early development. "For since each organ
becomes what it is only through the manner of its development, its true
value can be recognised only from its method of formation. At present,
we form our judgments by an undefined intuition, instead of regarding
each organ merely as an isolated product of its fundamental organ, and
discerning from this standpoint the correspondences and dissimilarities
in the different types" (p. 233). Parts, therefore, which develop from
the same "fundamental organ," and in the last resort from the same
germ-layer, have a certain kinship, which may even reach the degree of
exact homology.

Now since the mode of development in each type is peculiar to that type,
organs of the same name in different types must not necessarily be
accounted homologous, even if they correspond exactly with one another
in their general _functional_ relations to the rest of the organs. Thus
the central nervous system of Arthropods must not be homologised with
the central nervous system of Vertebrates, for it develops in a
different manner. So, too, the brain of Arthropods or of Mollusca is not
strictly comparable with the brain of Vertebrates. Again, the air-tubes
or tracheae of insects are, like the trachea and bronchi of many
Vertebrates, air-breathing organs. But the two organs are not
homologous, for the air-tubes of Vertebrates are developed from the
alimentary tube ("fundamental organ" of the alimentary system, developed
from the vegetative layer), while the air-tubes of insects arise either
by histological differentiation, or by invagination of the skin (p.
236). Organs can be homologous only within the limits of the big groups;
there can be no question of homology between members of different types.

The development of plants, like the development of animals, i