THEORIES OF EVOLUTION 131 



tion in successive generations. That this possibility 

 exists cannot be denied, but it is no more than a 

 possibiHty under the usual uncritical acceptance 

 of mutations as spontaneous changes. Hence nat- 

 ural selection plus mutation is of little more value 

 in evolutionary theory than either process alone. 

 It is more than probable that both are effective 

 processes in the modification of organisms, but 

 scarcely possible that they are an adequate expla- 

 nation of organic evolution generally. 



In order to meet these conditions the idea of a 

 tendency to produce mutations progressively in a 

 given direction has been advanced, which is closely 

 akin to the theory of orthogenesis, an uncertain 

 quantity in the field of evolution. Orthogenesis 

 has been a distinctly vitalistic theory, as in the 

 hands of Nageli, depending upon an internal di- 

 recting force which was supposed to cause develop- 

 ment in a given direction. It has also been a 

 distinctly Lamarckian theory involving the progres- 

 sive response of a succession of generations to the 

 same environmental conditions. To these views 

 may be added the idea of progressive mutations. 

 The word applies very well. to any of the phylo- 

 genetic series which palaeontology has given us, 

 but it is descriptive rather than explanatory. As 

 Newman has stated, "orthogenesis is not a causo- 

 mechanical theory of evolution at all." ^^ 



^2 Evolution, Genetics and Eugenics, Revised Edition, p. 36, 1925. 



