ORTHOGENETIC THEORIES 561 



in 1791; of the hornless or polled Hereford cattle from a single 

 calf born at Atchinson, Kansas, in 1889; of tailless cats and dogs; 

 and of hairless cattle, dogs, mice, and horses are further examples of 

 Evolution by Mutation. Such an evolutionary process supple- 

 ments rather than supplants Evolution by Natural Selection in the 

 scheme of evolutionary causation, and such was de Vries' concept 

 of its relationship. It is more fundamental than the Theory of 

 Selection, since it deals with the underlying phenomena of varia- 

 tion and heredity. Natural Selection appears in a more limited 

 role, but, on the other hand, it is relieved of the criticism that the 

 fluctuating variations emphasized by Darwin can not be the means 

 of evolutionar}^ change, because not inherited. 



Objection to the Mutation Theory has been raised on the ground 

 that mutations are not of sufficient frequency in nature to be the 

 material of evolution, and again that the limited number of indi- 

 viduals exhibiting a particular mutation would have small chance 

 of leaving enough descendants to establish a population. The 

 answer to this criticism is that any mutation seems likely to 

 appear not once but a number of times in any generation, and 

 in successive generations. Again, it was beheved by de Vries that 

 species have mutating periods, lasting, perhaps, thousands of 

 generations, during which numerous mutations appear and 

 hence evolution proceeds rapidly. A similar idea has been ex- 

 pressed by paleontologists. It may also be supposed that muta- 

 tions, which are due to certain changes in the germplasm, are 

 caused by environmental influences and that similar influences 

 may produce similar results again and again, thus offering repeated 

 opportunities for selection. It has already been explained 

 that the cases actually studied by de Vries now seem to be exam- 

 ples of unusual combinations occurring according to the ]Mendelian 

 laws of heredity and not true mutations. The work of de Vries, 

 therefore, occupies a curious position in that it brought to light 

 principles that seem to apply elsewhere but not in the cases in 

 which they were " discovered." 



Orthogenetic Theories 



The Origin of Variations by Orthogenesis. — Throughout the 

 preceding sections the origin of variations that are inherited has 

 been emphasized as the basic phenomenon to be explained in any 



