The Mechanics Of Development

5. The minds of the supporters of the mechanical theory had still to move

along a fifth line in order to solve the riddle of the development of the

living individual from the egg, or of the germ to its finished form, the

riddle of morphogenesis. They cannot assume the existence of "the whole"

before the part, or equip it with the idea of the thing as a spiritus

rector, playing the part of a metaphysical controlling agency. Here as<
r />
elsewhere they must demonstrate the existence of purely mechanical

principles. It is simply from the potential energies inherent in its

constituent parts that the supply of energy must flow, by means of which

the germ is able to make use of inorganic material from without, 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)