CHARLES DARWIN—WEISMANN. 445 
viously been disregarded. In this no reproach is implied to the 
many excellent observers of that period. No one can possibly 
observe everything that takes place; for instance, in the develop- 
ment of an animal, each notes only what seems to him to have some 
significance, whether he is able to interpret it or not. We do not 
work with our eyes alone; we must think at the same time. 
But I need not dwell longer on the manner in which the Dar- 
winian theory gained over the scientific workers of all countries, 
and penetrated deeply even among the laity. We have all had some 
personal experience of it, for the triumph of the theory of evolution 
has not long been won. A few words may be necessary as to why it 
was won so easily and so completely. 
This was due in part to the enormous and increasing mass of facts 
in support of it, but mainly to Darwin’s discovery of a principle 
capable of explaining transformations, in so far at least as these 
are “adaptations”—the principle of selection. Lamarck, too, had 
thought out a principle of explanation—the use or disuse of parts— 
*but it was obviously insufficient to explain evolution as a whole, 
since it could only apply to actively functional organs. 
The discovery of the principle of selection is the greatest achieve- 
ment of Charles Darwin and his contemporary, Alfred Wallace, and 
it alone, in my opinion at least, affords a secure basis for the theory 
of evolution. It reveals to us how the apparently impossible becomes 
possible, how what is adapted to its purpose can have arisen without 
the intervention of a directing power. 
The principle of selection shows us how the thousands of adapta- 
tions in living beings which arouse our constant admiration may have 
arisen in a purely mechanical way. And they must necessarily have 
done so if the evolution of the living has resulted from the same 
forces and laws as the not living; in other words, if, in explaining 
natural phenomena, we can leave out of account altogether any forces 
outside of or beyond nature. The principle of selection enables us 
to do this, and therein lies its far-reaching significance. It is, I 
believe, the discovery of this principle that will make the name of 
Darwin immortal. Wallace, too, deserves a full share of the credit, 
although he did not base his theory on such a broad foundation of 
facts, and did not apply it in so many directions. 
This principle is fully developed in “The Origin of Species by 
Means of Natural Selection,” as, indeed, the title of the book shows. 
It might be thought that the publication ofthis book finished the 
labors of the hermit of Down, but this was not the case; it was 
followed by the richest creative period of his life. Between 1860 
and his death in 1882 he issued a whole series of works, small and 
large, each of them based upon numerous observations and experi- 
ments, and most of them containing wholly fresh associations of 
