to
assimilate it and increase its own substance, and, by using it up, to
maintain and increase its power of work, to break up the carbonic acid of
the atmosphere and to gain the carbon which is so important for its vital
functions, to institute and organise the innumerable chemico-physical
processes by means of which its form is built up. Purely as a consequence
of the chemico-physical nature of the germ, of the properties of the
substances included in it on the one hand, and of the implicit structure
and configuration of its parts, down to the intrinsic specific undulatory
rhythm of its molecules, it must follow that its mass grows exactly as it
does, and not otherwise, that it behaves as it does and not otherwise,
duplicating itself by division after division, and by intricate changes
arranging and rearranging the results of division until the embryo or
larva, and finally the complete organism, is formed.

An extraordinary amount of ingenuity has been expended in this connection,
in order to avoid here, where perhaps it is most difficult of all, the use
of "teleological" principles, and to remain faithful to the orthodox,
exclusively mechanical mode of interpretation. To this category belong
Darwin's gemmules, Haeckel's plastidules, Naegeli's micellae, Weismann's
labyrinth of ids, determinants, and biophors within the germ-plasm, and
Roux's ingenious hypothesis of the struggle of parts, which is an attempt
to apply the Darwinian principle within the organism in order here also to
rebut the teleological interpretation by giving a scientific one.(66)

Heredity.

6. With this fifth line of thought a sixth is associated and intertwined.
The problem of development is closely bound up with that of "heredity." A
developing organism follows the parental type. The acorn in its growth
follows the type of the parent oak, repeating all its morphological and
physiological characters down to the most intimate detail. And the animal
organism adds to this also the whole psychical equipment, the instincts,
the capacities of will and consciousness which distinguish its parents.
The problems of the fifth and sixth order are closely inter-related, the
sixth problem being in reality the same as the fifth, only in greater
complexity.

A step towards the mechanical solution of this problem was indicated in
the "preformation theory" advanced by Leibnitz, and elaborated by Bonnet.
According to this theory the developing organism is