14 THE NEWEST THEORIES CONCERNING EVOLUTION SEC. 



which would be no less inexplicable than atavism, apart from 

 Weisinann's theory, seems to be. 



According to my ideas, it is not that sexual mixing of 

 characters, together with adaptation, determines the modifica- 

 tion of forms, but rather that sexual differentiation itself is 

 due to acquired and inherited characters. 



But it is not my purpose at this point to bring forward all 

 the arguments on which I rest my opposition to Weisrnann's 

 theory ; these arguments will be further developed in subse- 

 quent pages. 



A theory essentially opposed to Weismann's regarding the 

 causes of the modification of organic forms, in other words, the 

 origin of species, has been set forth by Nageli under the title 

 " Mechanico-physiological Theory of the Doctrine of Descent." l 



According to him, internal causes depending on the nature 

 of the organic substance effect the transformation of the 

 " strains " (individuals, species, families, etc.) in definite direc- 

 tions. Such " internal causes " must necessarily be supposed 

 merely on the ground that the modifications or variations of 

 the strains do actually take place in definite directions and 

 are not irregular. The internal causes effect a constant altera- 

 tion of the strains in definite directions "towards greater 

 perfection, that is, towards greater complexity." The strains 

 grow, as it were, towards greater perfection. Accordingly, 

 Nageli describes his theory of internal causes as the principle 

 of " improvement." " Superficial reasoners," he says, " have 

 pretended to discover mysticism in this. But the principle is 

 one of mechanical nature, and constitutes the law of persist- 

 ence of motion in the field of organic evolution." " Once the 

 motion of evolution is started it cannot cease, but must persist 

 in its original direction." 



By greater perfection, then, Xageli understands more com- 



1 C. v. Niigeli, Mechanisch-physidogische Abstammungslehre, Mtinchen und 

 Leipzig, 1884. 



