sly and freely. Thus the sensitive plant _Mimosa pudica_ droops
its feathery leaves when touched. Here, too, must be classed also all the
innumerable phenomena of Heliotropism, Geotropism, Rheotropism,
Chemotropism, and other tropisms, in which the sun, or the earth, or
currents, or chemical stimuli so affect a form of life--plant, alga, or
spore--that it disposes its own movements or the arrangements of its parts
accordingly, turning towards, or away from, or in an oblique direction to
the source of stimulus, or otherwise behaving in some definite manner
which could not have been deduced or predicted from the direct effects of
the stimulating factors. The upholders of the mechanical theory have
attempted to conquer this vast and mysterious domain of facts by seeking
to do away with the appearance of spontaneity and freedom, by
demonstrating in suitable cases that these phenomena of spontaneity and
the like would be impossible were it not that the potential energies
previously stored up within the organism are liberated by the stimulus.
Thus the effect caused is not equivalent to the stimulus alone, but is
rather the resultant of the conditions given in the chemo-physical
predispositions of the organism itself, and in the architecture of its
parts, _plus_ the stimulus.

Directly associated with this property of irritability is another form of
spontaneity and freedom in living beings--the power of adapting themselves
to changed conditions of existence. Some do not show this at all, while
others show it in an astonishing degree, helping themselves out by new
contrivances, so to speak. Thus the organism may protect itself against
temperature and other influences, against injury, making damages good
again by self-repairing processes, "regenerating" lost organs, and
sometimes even building up the whole organism anew from amputated parts.
The mechanical interpretation must here proceed in the same way as in
dealing with the question of stimuli, applying to the development of form
the same explanations as are there employed. And just because this domain
does not lend itself readily to mechanical explanation, we can understand
that confidence in the sufficiency of this mode of interpretation grows
rapidly with each fresh conquest, when this or that particular process is
shown to be actually explicable on mechanical principles. Processes of
development or morphogenesis--which are among the most intricate and
difficult--are attack