222 THE AMERICAN NATURALIST [Vol. XLI 



to the association of species of plants and their varieties upon the 

 same ground, and states that when one form replaces another in 

 consequence of change of ecological conditions within the same 

 district, the replacing form is not related to the other in the closest 

 grade of affinity, but in some degree more remote. He clearly 

 recognizes the intimate relation of distributional studies to the 

 question of evolution. 



There is a little bit of evidence from Wallace (:00, p. 391). 

 He says he made inquiries of two experienced English botanists 

 to find whether well-defined varieties occupy areas to the exclusion 

 of the type and do not occupy the area or only a very small one 

 with the type. Only one such case was found in England. Wal- 

 lace's conclusion is that such varieties of plants occupying consider- 

 able areas to the exclusion of the type are not common. 



Asa Gray ('59, p. 193) expressed the following opinion: 

 "Whether capable of scientific explanation or not it is certain that 

 related species of phienogamous plants are commonly associated 

 in the same region or are found in comparatively approximate 

 areas, however large, of similar climate." 



The case of Draba verna L., is most interesting. As is well 

 known, about two hundred distinct species, or at least kinds, of 

 Draba have been distinguished within the limits of the original 

 I^inna^an species Draba verna. These numerous forms were 

 studied in cultivation by A. Jordan, and later by De Bary and 

 F. Rosen. They are found to come true to seed, and for this 

 reason are by these authorities spoken of as species. Their geo- 

 graphic distribution is discussed by both Jordan ('73) and Rosen 

 ('89, p. 613). The conclusion is that as a rule the forms which 

 resemble each other most are found in the same stations. The 

 joint occurrence of next related species is indeed a fact which 

 ])articularly impressed both of these writers. Rosen thinks that 

 it is very unlikely that these closely related species originated 

 separately and by chance came to be associated in the fashion in 

 which they are now found. Such an explanation might serve, he 

 says, if one or two cases only were to be explained ; but it becomes 

 absurd when we consider that the concomitance of next related 

 forms is wide-spread. Rosen ends his account of this group of 

 Draba species with a very clear statement of the mutative origin 



