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 Ins 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 Nageli nor 

 Weismann, neither De Vries nor Roux, has done this. Nageli, in his 

 Mechanisch-Physiologische Theorie der Abstammungslehre 1 , 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 2 , 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 3 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 



i Munich, 1884. 2 Die Mutationstheorie, Leipzig, 1903. 



5 Der Katnpf der Theile im Organismus, Leipzig, 1881. 



