88
The Presidential Address.
been subject to identical conditions throughout the entire
course of their development, we see that perfect identity in
the result would be opposed to everything we know of natural
agencies. But variation is merely the absence of identity, and
therefore requires no further explanation; neither do the
diverse amounts of variation, for they correspond to the
countless diversities of conditions to which animals have
been exposed, either during their own development or that of
their ancestors."31
It would be doing an injustice to the memory of Charles
Darwin were I to leave upon your minds the impression that
he regarded natural selection as the sole cause of species
transformation. In the 'Origin' certain other important
factors, such as the direct action of external conditions, use
and disuse, compensation and economy of growth, correlated
variability, &c, are discussed at some length. The principle
of sexual selection — by which the difference in colour,
ornamentation, weapons, &c, of such frequent occurrence
between the sexes throughout the animal kingdom, is explained
on the view that either the male or female in each species
has throughout a long series of generations selected variations
having these characteristics — was also laid down in the
'Origin,' and afterwards elaborated in great detail in the
'Descent of Man,' the main portion of this work being
devoted to the establishing of this principle. But after
giving due weight to these auxiliary factors the chief share
in the work of modification is still assigned to natural
selection, although the candid avowal is made that there may
be other unknown processes in operation. The truly philo-
sophical side of Darwin's character is perhaps nowhere better
shown than in the admission that he had not explained
everything—that his system was not to be taken as final.
He has not given us a complete theory of things in the sense
of ancient scholasticism, which for the bread of knowledge
substituted a stone that stood as a stumbling-block in the
way of further advancement. Darwin has demonstrated a
vera causa of organic evolution, but he has done even more
31 A. K. Wallace, 'Nineteenth Century,' Jan., 1880, p. 105.