as a modification of, the germ-plasm. The
germ-plasm can increase indefinitely in the lapse of generations,
increase of the somatoplasm is limited.

When a new individual develops, a certain portion of the germ-plasm
of the egg is set aside and remains unchanged in structure. This,
increasing in quantity, forms the reproductive elements for the next
generation. The germ-plasm, which does not form the whole of each
reproductive element, but only a part of the nucleus, is thus an
exceedingly stable substance. And there is a just as real continuity
of germ-plasm through successive generations of volvox, or of any
higher plants or animals, as in successive generations of protozoa.

In certain plants there is an underground stem or rootstock, which
grows perennially, and each year produces a plant from a bud at its
end. This underground rootstock would represent the continuous
germ-plasm of successive generations; the plants which yearly arise
from it would represent the successive generations of adult
individuals, composed mainly of somatoplasm. Or we may imagine a
long chain, with a pendant attached to each tenth or one-hundredth
link. The links of the chain would represent the series of
generations of germ-cells; the pendants, the adults of successive
generations.

But any leaf of begonia can be made to develop into a new plant,
giving rise to germ-cells. Here there must be scattered through the
leaves of the plant small portions of germ-plasm, which generally
remain dormant, and only under special conditions increase and give
rise to germ-cells.

A large part of the germ-plasm of the fertilized egg is used to give
rise to the somatoplasm composing the different systems of the
embryo and adult. Weismann's explanation of this change of
germ-plasm into somatoplasm is very ingenious, and depends upon his
theory of the structure of the germ-plasm; and this latter theory
forms the basis of his theory of evolution. It would take too long
to state his theory of the structure of germ-plasm, but an
illustration may present fairly clear all that is of special
importance to us.

The molecules of germ-plasm are grouped in units, and these in an
ascending series of units of continually increasing complexity,
until at last we find the highest unit represented in the nucleus of
the germ-cell. This grouping of molecules in units of increasing
complexity is like the grouping of the men of an army in companies,
regiments, brigades,