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 he 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. 
