ssion of the notion of these hypothetical
units of organic existence, see Weismann's Germinal Selection,
(Open Court Publishing Co., Chicago, 1896), especially the foot
note, page 230.

Naegeli's _Laws of Evolution_ are also worth special notice. As stated in
the body of _Abstammungslehre_ they are as follows:

1. Asexual reproductive cells which arise by division, remain united and
become tissue cells.

2. Asexual reproductive cells which arise by budding, instead of
separating, become cell branches or branched cell threads.

3. Reproductive cells which arise by free cell formation become bodies
which form a part of the cell contents.

4. Parts of a plant which arise by differentiation lie side by side and
form a body of web-like or tissue-like structure.

5. A definite and previously limited growth continues, or a definite
formation of parts of an ontogeny which has previously been present but
once, is repeated. (Ampliation.)

6. The parts of an ontogeny become dissimilar, since the functions which
were previously united become differentiated and since new dissimilar
functions are produced in the various parts. This differentiation is
either one of space between the parts of the ontogeny that appear near
each other, or one of time between those that are derived from each
other.

7. Parts which have become dissimilar by differentiation undergo a
reduction, in which the intermediate forms are suppressed and at last
only the qualitatively dissimilar forms with qualitatively dissimilar
functions remain.

8. The environment in which plants live operates in different ways,
directly as a stimulus or indirectly as a felt necessity and by this
means lends to their forms and activities a definite expression of time
and place, and thus brings about different adaptations. These become
permanent through heredity, but are again gradually lost if other
adaptations supersede them.

Laws 1 to 4 may be expressed as one--the law of combination: Similar
parts that are wholly or partly separated have the tendency to unite
more and more completely and intimately into one continuous tissue.

The laws of ampliation (5), differentiation (6), and reduction (7), may
be summarized in one as follows: While increasing in size the similar
parts of an ontogeny become internally dissimilar and the dissimilarity
increases as the transition forms of the dissimilar parts vanish. Hence
only the extreme forms remain.

*