of
successive generations are all independent; the ontogenetic period
consists of a cycle of generations of unicellular plants. Later the cell
generations of an ontogeny are united by parts into plant individuals;
the ontogenetic period consists of a cycle of multicellular and
unicellular, or only of multicellular plant generations. If all the cell
generations of an ontogenetic period have been united into a single
individual, the successive plant generations are alike and alternation
of generations has ceased.

The unlikeness of the generations arises either from inner causes of
temporary differentiation alone, or by temporary differentiation which
receives a definite imprint by the change of seasons. But in the latter
case the characteristic of adaptation is again lost in the course of the
phylogeny and alternation of generations follows then without regard to
the season. If the given adaptation is united in the lower plants with
alternation of generations during the ontogenetic periods, one of the
unlike plant generations is repeated an indefinite number of times
(repetitional generation), while the other unlike plant generation
appears only once and then at the beginning of the resting stage and
remains latent in the form of a resting spore till the beginning of the
next period of generation. With this peculiar transition generation,
which has arisen in the lower stages asexually, and in the following
higher stages by the union of a male and a female cell, and which hence
is hermaphrodite, there are generally associated later two other single
generations--_viz._, a generation preceding and one following the
hermaphrodite, the former as a sex-producing generation, the other as a
sex-produced generation.

The phylogenetic significance of the alternation of generations consists
in its representing a transition stage from the unicellular to the
simpler multicellular and from the latter to the more complex
multicellular plants. The plant generations of any phylogenetic stage
increase by ampliation, become unlike by differentiation in time
(alternation of generations), and unite in a plant individual, whose
unlike ontogenetic stages correspond to the unlike plant generations of
the earlier ancestral series.

22. MORPHOLOGY AS THE SCIENCE OF PHYLOGENY.

All organic phenomena belong, according to their causes, to two
different classes: (1) Those belonging to one group are the results of
external influences in each o