Darwin’s Successors 141 
finely adapted structure of the animal or plant body be formed 
unless it was built on a preconceived plan? It thus enables us to 
dispense with the teleology of the metaphysician and the dualist, 
and to set aside the old mythological and poetic legends of creation. 
The idea had occurred in vague form to the great Empedocles 
2000 years before the time of Darwin, but it was reserved for modern 
research to give it ample expression. Nevertheless, natural selection 
does not of itself give the solution of all our evolutionary problems. 
It has to be taken in conjunction with the transformism of Lamarck, 
with which it is in complete harmony. 
The monumental greatness of Charles Darwin, who surpasses 
every other student of science in the nineteenth century by the 
loftiness of his monistic conception of nature and the progressive 
influence of his ideas, is perhaps best seen in the fact that not one of 
his many successors has succeeded in modifying his theory of descent 
in any essential point or in discovering an entirely new standpoint 
in the interpretation of the organic world. Neither Nigeli nor 
Weismann, neither De Vries nor Roux, has done this. Niageli, in his 
Mechanisch-Physiologische Theorie der Abstammungslehre', which 
is to a great extent in agreement with Weismann, constructed 
a theory of the idioplasm, that represents it (like the germ-plasm) as 
developing continuously in a definite direction from internal causes. 
But his internal “principle of progress” is at the bottom just as 
teleological as the vital force of the Vitalists, and the micellar 
structure of the idioplasm is just as hypothetical as the “dominant ” 
structure of the germ-plasm. In 1889 Moritz Wagner sought to 
explain the origin of species by migration and isolation, and on that 
basis constructed a special “migration-theory.” This, however, is 
not out of harmony with the theory of selection. It merely elevates 
one single factor in the theory to a predominant position. Isolation 
is only a special case of selection, as I had pointed out in the fifteenth 
chapter of my Natural history of creation. The “mutation-theory” 
of De Vries’, that would explain the origin of species by sudden and 
saltatory variations rather than by gradual modification, is regarded 
by many botanists as a great step in advance, but it is generally 
rejected by zoologists. It affords no explanation of the facts of 
adaptation, and has no causal value. 
Much more important than these theories is that of Wilhelm 
Roux® of “the struggle of parts within the organism, a supple- 
mentation of the theory of mechanical adaptation.” He explains 
the functional autoformation of the purposive structure by a 
combination of Darwin’s principle of selection with Lamarck’s idea 
1 Munich, 1884. 2 Die Mutationstheorie, Leipzig, 1903. 
® Der Kampf der Theile im Organismus, Leipzig, 1881. 
