eduction._--The cells, and, in general, the parts of plants which lie
near each other in space or follow upon each other in time, are always
alike in the lower stages. By differentiation they become unlike, so
that the sum of the functions which at first fall to the lot of all
parts without distinction now is shared among the individual parts. By
this means each part can perform its own special function so much the
better. Differentiation is repeated in the course of the phylogeny,
since at first all parts of an ontogeny diverge into two or more parts,
then the parts of these parts divide again, etc. Along with this process
of division another process is always active, which, as it were,
prepares the way for the former, namely, ampliation, by virtue of which
the growth of the whole ontogeny or of single stages of it undergoes a
quantitative increase, so that an organ acquires a greater number of
cells, and an individual a greater number of organs. After this increase
in number of parts in a stage of ontogeny, differentiation follows as
far as the nature of the functions permits, by the parts most separated
passing into each other by intermediate gradations. By the further
phylogenetic process of reduction the intermediate forms are suppressed.
At last only the extreme products of differentiation lie near each other
in space or follow upon each other in time; and these products are as
limited in quantity and number as possible.

Along with the above named phylogenetic processes, which take place by
the automatic increase of the idioplasm, external influences are always
active. These lend to the organism at times a local stamp corresponding
to its environment, and follow the law of adaptation.

21. ALTERNATION OF GENERATIONS IN RELATION TO PHYLOGENY.

Since the simplest plants are cells and the more complex ones are formed
from cells, a whole phylogenetic line may be regarded as a series of
cell generations following one after another. In the lowest forms all
cell generations are like each other; in all others they show
differences which become continually greater and more numerous. Thus
alternation of generations in cells exists, because the successive
generations become more and more complicated at each succeeding period.
Among these periods the ontogenetic period or ontogeny embraces all
generations from one cell to the return of the exactly similar kind of
cell. In the lowest forms of cell differentiation the cells