390 
AMERICAN JOURNAL OF BOTANY 
[VoI. io, 
types of plasticity are measurably limited. The tendency toward a cer¬ 
tain succession of phases sometimes, no doubt, results from limitations 
imposed by the conditions of the particular present phase. Thus, a plasmo- 
dium or a multinucleate cyst, of whatever species, can not, or at least 
usually does not, return to the uninucleate condition except by a division 
into uninucleate cells, in which process the identity of the mother cell 
disappears. Here a different course of events is clearly difficult or impos¬ 
sible. But the limitations noted upon the sequence of quiescent, flagellate, 
and amoeboid phases have a less obvious physical basis. Such limitations, 
like the potentialities whose expression they affect, are inherited. Like 
the potentialities, they seem to have their basis in the fundamental organi¬ 
zation of the living matter. 
Other heritable characters that may be considered directing or limiting 
tendencies are polarity and the various specific types of cellular symmetry. 
In the same category perhaps belong some of the “ inhibiting ” and “ lethal ” 
factors of genetic analysis. It appears, then, that the fundamental organi¬ 
zation of living matter, besides giving rise to a great variety of potentialities, 
likewise in some degree limits and directs the order of expression of those 
potentialities. The question is still open whether the living matter, apart 
from the interaction of the environment, is able to do more than limit and 
direct—namely, to initiate the expression of any of its own potentialities. 
Especially characteristic of living matter is one potentiality of fat- 
reaching significance—that of undergoing changes in its fundamental 
features. From time to time, and under almost entirely unknown condi¬ 
tions, the specific character of the living matter is altered; some of its poten¬ 
tialities may be modified, some lost, or new ones acquired; and thus a new 
race is born. The possibility of changes in fundamental organization is 
limited; evolution can not move in any conceivable direction. The direc¬ 
tions in which it may move are themselves determined by the constitution 
of the living matter. A question now arises paralleling that already asked 
concerning the expression of specific potentialities: How far are evolution¬ 
ary changes themselves responses to stimuli, and how far the necessary 
result of the cell’s constitution? To what extent, if at all, would new 
races arise if the relatively persistent features of the organization of living 
matter were uninfluenced by the environment? This question has been 
variously answered, but it is doubtful if any of the answers is more than an 
interesting guess. 
II 
Each of the types of evolutionary change just suggested—loss, gain, 
and modification of potentialities—is illustrated by flagellates that have 
become adapted to a strictly or mainly holozoic, saprophytic, or parasitic 
existence. Such highly developed parasites as the Trichonymphidae, for 
example, have lost the potentialities concerned in photosynthesis. Cor- 
