86 The Presidential Address.
sometimes spoken of as taking place in every direction ; that
is to say, it is supposed that every species presents an infinite
number of variations for natural selection to act upon. Thus
it may be fairly asked, why under these circumstances ex-
tinction should ever take place ? If variability is infinite,
why should there not be a capability on the part of any
species to meet any contingency—to become modified in any
direction ? In reply to this question, it may be stated that
the range of variability required by Darwin's theory, although
wide, is not and cannot be unlimited. There are restraining
forces within the organism. The force of heredity, by which
the latter tends to retain not only the characters of its imme-
diate parents, but likewise those of more remote ancestors,
determines the physical constitution of the organism, and
checks unlimited variability. The delicate balance of in-
ternal forces constituting the physiological equilibrium of the
organism cannot admit of being, displaced in every conceivable
direction—there can be only a certain range of variability
possible to each species. An organism may be compared to
a bell, which, although capable of vibrating in a very great
number of ways and of giving out a great number of notes
according to the way in which it is struck, nevertheless
cannot be made to give out an infinite number of notes,
because its molecules are constrained to vibrate in certain
definite ways only by virtue of its physical construction. In
every case of species-transformation there are thus involved
two factors, "the nature of the organism, and the nature of
the conditions."29 It is to Professor Weismann that we owe
the first full recognition of the important part played by the
organism itself in the process of evolution, but it must be
borne in mind that the part thus played is quoad modification,
a purely passive one.
In connection with the foregoing considerations, there
arise the questions of the causes of variability and the extent
to which the latter takes place in particular species. It is
very much to be regretted that Darwin was compelled by failing
health to abandon his intention of discussing the subject of
29 'Origin,' 6th ed., p. 6.