3 16 cook 



The determinant theory of Nageli, as already indicated, 

 ascribed changes to an internal "principle of perfection" of 

 heredity, which conducted the evolution of a species in a definite 

 direction. There was no need, in this view, of showing any 

 direct connection with the environment. Selection was applied 

 to a species as a whole, to preserve or to eliminate, but it was 

 not thought of as actuating evolution or as conducting it in 

 adaptive directions. 



The determinant theories of Weismann and his followers 

 may be described as hybrids between the doctrines of Nageli 

 and those of Darwin and Lamarck. They predicated a cel- 

 lular mechanism of heredity for conducting the process of 

 evolution, but supposed that this mechanism could be actuated 

 or affected by environmental influences and compelled in this 

 way to carry the species in directions of adaptation. 



Darwin, in his theory of pangenesis, assumed that all parts 

 of the body of the parent contribute materials to the germ-cells 

 and hoped thus to explain how characters acquired from the 

 environment might be passed on to succeeding generations. 

 Weismann denied the inheritance of acquired characters, but he 

 nevertheless repeated Darwin's attempt at providing for the 

 inheritance of environmental influences, because it appeared 

 impossible without this to construct a theory of environmental 

 causation and explain the facts of selection and adaptation. 



Weismann was well aware that his theory of determinants 

 was so complex as to appear improbable, but he defended it 

 with persistence on the ground that it was the only way in 

 which heredity could be understood. Unfortunately, the vast 

 complexity of ideas does not explain the facts of organic descent, 

 but only adds to them an even more mysterious hypothetical 

 field. Moreover, the data of environmental relations do not 

 accord any better with the Weismannian than with the Dar- 

 winian hypothesis. Experiments have not shown that there is 

 any close, constant or definite relations between environment 

 and heredity. The most that can be claimed is that the environ- 

 ment, in some manner still quite unexplained, may sometimes 

 induce an instability, or tendency to stumble and fall from the 

 normal hereditary pathway of the type. 



