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 lias demonstrated a 
vera causa of organic evolution, but he has done even more 
81 A. E. Wallace, ‘Nineteenth Century,’ Jan., 1S80, p. 105. 
