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logic of which was irresistible, assuming the premises to 
be true. It left untouched some of the greatest difficulties 
associated with Organic Evolution—the origin of life, the 
laws of variation, the mysteries of reproduction. It ex 
plained all by assuming that the variations necessarily 
associated with reproduction were the source of heritable 
variations—that selection in nature acted in the same 
fashion, for the most part, as selection in art; and that 
similar results might be looked for—nay, still greater, 
inasmuch as the works of nature are greater than the 
works of man. 
The third reason for the success of this theory lies in the 
fact that it was promulgated by men who were not mere 
theorists, but careful observers of the actual world of 
nature. But experience warns us that scientific observers 
of the calibre of Darwin and of Wallace, who have done 
good work as careful observers of nature, have sometimes 
promulgated scientific theories which have not been 
sustained. 
Another reason is found in the elasticity of the terms 
employed and the indefiniteness of the definitions which 
are often given. It prevailed moreover because the effect 
of its promulgation was to depreciate the influence of other 
factors of Organic Evolution. But I venture to go a step 
further back, and to say that the theory of Natural 
Selection has prevailed because scant justice has been 
done to the arguments for Organic Evolution, which were 
used before the date of the publication of The Origin of 
Species. 
It is candidly admitted by believers in Natural Selec 
tion that, if it should be proved that Natural Selection 
was not after all a law of nature, it would not affect the 
arguments for Organic Evolution ; it would only imply 
that some factors, other than Natural Selection, had