Xo.Tll] THE THEORY OF ORTHOGENl 



in the same direction in the four groups, when one con- 

 siders the great variety of conditions with which they 

 are associated. To explain this definite development on 

 the basis of orthoselection it would he necessary to adopt 

 the point of view that each form in each group found it of 

 advantage in the struggle for existence in its particular 

 environment to become more dwarfed than its immediate 

 ancestor, which is to my mind unthinkable when one con- 

 siders that we have in this genus four groups that push 

 out in all directions from the center of origin, into desert, 

 grassland and forest regions, in the tropical and tem- 

 perate zones, and yet in every case the modifications asso- 

 ciated with each region are practically of the same nature 

 and non-adaptive. We meet the same difficulties if we 

 attempt to apply the mutation theory to explain this 

 definite evolution, for, while de Vries 's mutants arise 

 suddenly and are definite steps in new directions, he 

 states in regard to the accumulation of characters : 4 



It is not by mere chance that the variations move in the required 

 direction. They do go, according to Darwin's view, in all directions, 

 or at least in many. If these include the useful ones, and if this is 

 repeated a number of times, cumulation is possible; if not, there is 

 simply no progression, and its type remains stable through the ages. 



It seems from this that in order to explain the evolution 

 of a group by a series of modifications more or less similar 

 in kind the mutation theory of de Vries is forced to fall 

 back upon natural selection. If I have rightly inter- 

 preted the conditions, I believe that natural selection, with 

 or without the assistance of the mutation theory, fails 

 as an explanation of the definite evolution of this genus, 

 and that we have here an example of true orthogenesis, 

 i. e., progressive modifications in each group without the 

 aid of natural selection. 



The nature of the variations is very interesting iri 

 several ways, and throws further light on the problem, 

 for, although one must be cautious of forming conclu- 

 de Vries, H., "Species and Varieties, Their Origin by Mutation," 



