Recent Developments in Heredity and Evolution 15 



genesis. The variations utilized in the preceding explana- 

 tions, both the smaller ones used by natural selection and 

 the larger ones used by mutation, occur in every direction 

 from the parent form, the successful direction being deter- 

 mined by natural selection. This has been called indeter- 

 minate variation. In tracing the evolution of great groups, 

 howe^'er, it becomes clear that the most important varia- 

 tions occur in certain definite directions, which ha^'e been 

 maintained persistently throughout all possible changes of 

 condition. For example, the history of such a group as 

 gymnosperms shows a tendencj- to vary in certain definite 

 directions that has persisted from the early Paleozoic to 

 the present time. In other words, there is much to indicate 

 that while variation may be indeterminate, there are also 

 certain definite lines that persist. The origin of new forms, 

 whether by natural selection or mutation or neither, as the 

 result of a persistent determinate variation, is called ortho- 

 genesis. It certainly removes one of the greatest difficul- 

 ties in the way of natural selection, and that is the beginning 

 and development of a structure that can be of advantage 

 only when it is completed. It satisfies also the many 

 known cases of excessi\'e development in certain directions, 

 a development that may be not only disadvantageous, but 

 even destructive. Even if determinate ^'ariation is accepted 

 as a fact, however, what determines the persistent variation ? 

 The answer to this cjuestion has resulted in many variations 

 of the theory of orthogenesis. 



It should be noted that natural selection, mutation, and 

 orthogenesis are not mutuall)' destructive. They all deal 

 with variations, and may all be operative in producing new 

 forms. Natural selection deals mth small variations which 

 are in every direction; mutation with large variations which 



