f vegetative (somatic) differences
and that no genetic change of factors was involved, for his conclusion that
the rapidity with which the effects gained by long selection might be
suddenly reversed when selection was reversed is hardly consistent with an
interpretation of the results based on changes in the "potencies" of the
factors present.

Equally striking are the interesting experiments that Jennings has recently
carried out with Difflugia (fig. 91). This protozoon secretes a shell about
itself which has a characteristic shape, and often carries spines. The
opening at one end of the shell through which the protoplasm protrudes to
make the pseudopodia is surrounded by a rim having a characteristic
pattern. The protoplasm contains several nuclei and in addition there is
scattered material or particles called chromidia that are supposed to be
chromatic in nature and related to the material of the nuclei, possibly by
direct interchange.

[Illustration: FIG. 91. Difflugia Corona. (After Cash.)]

When Difflugia divides, part of the protoplasm protrudes from the opening
and a new shell is secreted about this mass which becomes a daughter
individual. The behavior of the nucleus and of the chromidia at this time
is obscure, but there is some evidence that their materials may be
irregularly distributed between parent and offspring. If this is correct,
and if in the protozoa the chromatin has the same influence that it seems
to have in higher animals, the mode of reproduction in Difflugia would be
expected to give little more than random sampling of the germ plasm.

[Illustration: FIG. 92. Races of Difflugia. (After Leidy.)]

Jennings was able by means of selection to get from the descendants of one
original individual a number of different types that themselves bred true,
except in so far as selection could affect another change in them. In this
connection it is interesting to note that Leidy has published figures of
Difflugia (fig. 92) that show that a great many "types" exist. If through
sexual union (a process that occurs in Difflugia) the germ plasm
(chromatin) of these wild types has in times past been recombined, then
selection would be expected to separate certain types again, if, at
division, irregular sampling of the germ plasm takes place. Until these
points are settled the bearing of these important experiments of Jennings
on the general problem of selection is uncertain.

HOW DOES NATURAL SELECTION INFLUENC