leaf? Identical idioplasms should
always produce identical structures. Naegeli attempts to explain
this difficulty by attributing the different results to
different "conditions of tension and movement," i.e., a
dynamical difference between the idioplasms of the different
parts of the organism. (_Abstammungslehre_, p. 53.)

This idea of differences of structure being due to dynamic
rather than to material causes plays a considerable part in
Naegeli's theory, but is the point on which he speaks with least
certainty--in fact with a noticeable hesitation. He does not
clearly explain the phrase "conditions of tension and movement,"
nor does he give a convincing explanation of the known phenomena
as results of the action of dynamic influence.

Naegeli is not the only one who posits dynamic rather than
material differences as to the basis of diversities of
structure. More recently, Cope has built up a system of
evolution founded largely on this idea.--_Trans._

The continued phylogenetic formation of the threads of idioplasm takes
place by growth in the cross section, which contains the sum of all the
determinants and changes in general only when new rows of micellae are
intercalated. But the rows of micellae of the idioplasm join closely to
each other, according to their thickness, so that only rarely new rows
can enter, and then only at those definite places where the cohesion is
less strong and hence is overcome. The cohesion varies irregularly
because the configuration of the cross section, conformably to its
origin, is never regular; the disruptive tensions are brought about by
the unequal growth in length of the individual rows of micellae. Dynamic
influences have a decisive effect upon cohesion and disruptive tensions.
The groups of micellae of the configuration already obtained exercise
these dynamic influences upon each other; and these dynamic influences
can be modified by stimuli from without.

The idioplasm continually alters its configuration with its growth in
successive ontogenies, but comparatively very slowly, so that it makes a
minute advance from the germ of one generation to the germ of the next.
The summation of these increments of advance through a whole line of
evolution represents the race history of an organism, since the latter
is connected only by its idioplasm in unbroken continuity with the
micellar beginning of its race.

9. D