84 MUTATIONS, VARIATIONS, AND RELATIONSHIPS OF THE OENOTHERAS. 



species. If on the other hand it is a local relict of an old species, which once 

 had a wide distribution, it should present less variability than its wide ranging, 

 presumably newer relative. The only satisfactory evidence heretofore avail- 

 able for determining a point of this kind has been drawn from paleontological 

 sources. 



To the metaphysician who is striving to picture to himself the laws of vital 

 motion, the results of these statistical studies will give an altogether new view 

 of the relation between fluctuation and mutation. Rosa (1899) has published 

 a treatise in which he contends that there is a progressive lessening of varia- 

 bility during the progress of evolution, and he sees as the certain result of this 

 process the ultimate extinction of life. Plate (1904) takes issue with Rosa on 

 this point and maintains that there is no such law. The demonstration that 

 there is greater variability in new species, coupled presumably with a decrease 

 in variability as the species grows older, indicates that both Rosa and Plate 

 are partly right. It is now seen that although the individual species may 

 decrease in variability as it grows older, this decrease is compensated for every 

 time a new species springs into existence. Instead of mutations being the 

 cumulative results of ever-increasing fluctuation, they appear now as an initial 

 process of which fluctuation is in part an after effect. 



A detailed description of but a few of the species of Oenothera have been 

 given. Seeds and material have been brought in from all parts of America 

 east of the Rocky Mountains, and a consideration of this material gives 

 foundation for the conclusion that under the name biennis is included a swarm 

 of elementary species very closely related, but easily distinguishable when 

 grown side by side, having no intergrading forms. 



Even the imperfect knowledge we possess of the species, owing to the 

 scarcity of collections made of them, shows that they overlap in distribution 

 in the most haphazard manner. In the same way, the elementary species 

 comprised within cruciata interlock. In discussing the principles that govern 

 distribution, it is to be admitted that in accordance with their usual practice 

 many systematists would not allow specific rank to the forms in question, so 

 closely are they related. It will be in order, therefore, to take up the next 

 grouping of interest, that of biennis, and the allied species of oakesiana, parvi- 

 flora, muricata, and cruciata, which were accepted as varieties of biennis 30 

 years ago, but which are accorded specific rank at the present time. It is 

 found that biennis blankets nearly the entire range of these forms, and also 

 that the ranges of these forms overlap among themselves. It is to be seen, 

 therefore, that no matter what range of opinion may enter into the discussion 

 as to the rank of the forms of the evening-primroses of the eastern part of 

 North America, the thesis that the most nearly related species do not overlap 

 in distribution is unsupported. 



