MICHIGAN ACADEMY OF SCIENCE. 15 



l)een one of progressive dwarfing as it departed from the center o^ dispersal. 

 It is too often overlooked l)y students of evolution that natural selection 

 can cause directed evolution (orthoselection); for in order that it may do 

 so it is only necessary that there be an accruing advantage in the increased 

 development of a character. The characters must thus be utilitarian, 

 however, which is apparently not true in this case. Moreover, it would 

 certainly be taxing the theory to make it account for the continued develop- 

 ment in the same direction in the four groups, when one considers the great 

 variety of conditions with which they are associated. To explain this definite 

 development on the basis of orthoselection it would l)e 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 

 considers that we have in this genus four groups that push out in all direc- 

 tions from the center of origin, into desert, grassland and forest regions, in the 

 tropical and temperate zones, and yet in every case the modifications associated 

 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^'ries's mutants arise suddenly 

 and are definite steps in new directions, he states in regard to the accumula- 

 tion of characters:* 



It Is not by mere cliance that tlie variations move in the required direction. They do go, according 

 to Darwin'H 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 

 interpreted 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 in several ways,- and throws 

 further light on the problem, for, although one must be cautious of forming 

 conclusions on this subject without the controlling evidence of experimenta- 

 tion, certain general relations seem to be apparent. 



I have stated that the variations in each form fluctuate about a mean, 

 but that this mean varies, approaching in the intermediate region that which 

 characterizes the next form on its line of descent. That the different forms 

 also originated in this way is shown in the numerous instances where they 

 actually intergrade. This is essentially the idea of mutation (in the sense 

 of Waagen and Scott) or '.' phjdogenetic variations, " which is not to be dis- 

 tinguished from individual variation by any character of ciuality or quantity, 

 but by the fact that it pursues a determinate direction by the gradual 

 shifting of the normal type. Conn^ states that: 



It should be noticed that these considerations in regard to variations along definite lines have less 

 significance in connection with such characters as can be supposed to advance by general averages. 

 Some organs have been advancing in definite directions for long generations, but if the advance consists 

 of an increase or decrease in size of the organs there is not needed any law of determinate variation 

 to explain the matter. If it be an advantage to have an organ increase in size, and if variations in 

 this organ occur around an average type, then without-any necessity of supposing a special law directing 

 variations, we can understand how natural selection will continue to increase the size of the organ in 

 question. 



All this is very true, for it is only orthoselection working on the average 

 type, but as selection seems to be debarred in this case, the characters being 



^De Vries, H., "Species and Varieties, Their Origin by Mutation," p. 572. 

 *Conn, H. W., "The Method of Evolution," p. 146. 



