gans in ferns
can be interrupted, so that, in spite of the presence of male organs,
fertilization is impossible; at the same time, the prothallia are
enabled in weak light to grow feebly and to put out small asexual
processes, which in the presence of bright light become normal
prothallia. Similarly, the development of sexual organs in algae
is dependent on a certain intensity of light, and the plant remains
sterile if the light is diminished below a certain point. (G. Klebs,
_Ueber einige Probleme der Physiologie der Fortpflanzung_, pp.
13-16.)]

[Footnote 16: E. Maupas, "Theorie de la sexualite des Infusoires
cilies," _Comptes rendus_, Vol. CV, pp. 356ff.]

[Footnote 17: The extinction took place at about the 330th generation
in _Onychodromus grandis_, at about the 320th generation in
_Stylonichia mytilis_, at about the 330th generation in _Leucophrys
patula_, and at about the 660th generation in _Oxytricha_
(indeterminate). (Maupas, _loc. cit._, p. 358.)]

[Footnote 18: Maupas, _loc. cit._, p. 358. Later investigations have
tended to discredit Maupas' experiments as a whole by showing that the
Infusorians with which he experimented can be kept alive indefinitely
by a change of diet, without the aid of sexual conjugation. This
merely confirms the view, however, that abundant nutrition and
crossing are alike favorable to health: "We must admire the skill of
the investigator who was able to keep his colonies alive for months
and years under such artificial conditions, but we may venture to
doubt whether the fate of extinction which did ultimately overtake
them was really due to the absence of conjugation, and not to the
unnaturalness of the conditions." A. Weismann, _The Evolution of
Theory_, Vol. I, p. 329.

Since the above was written, Calkins has made a series of new
experiments, the results of which differed in several respects from
those yielded by Maupas' experiments. When his infusorian cultures
began to grow weaker, as happened frequently and at irregular
intervals, he was always able to restore them to more vigorous life by
a change of diet, and especially by substituting grated meat, liver,
and the like for infusions of hay. Certain salts too, had the same
effect; the animals became perfectly vigorous again. Calkins believes
that chemical agents, and especially salts, must be supplied to
the protoplasm from time to time. He reared 620 generations of
_Paramoecium_ without conjugation. But the 620th was we