1921] 
PAYSON—MONOGRAPH OF THE GENUS LESQUERELLA 127 
Schaueriana of south central Mexico; and finally the most 
specialized of the three, L. frigida, has been reported from Vene- 
zuela. It remains a remarkable probability that from one point 
should have come so many lines of development. 
Six species have been already mentioned as being rather 
widely isolated from any other representatives of the genus. 
It is quite worth while to examine these species in detail in order 
to determine if possible the significance of this isolation. There 
are at least two possibilities that would account for the location 
of these species. Either the continuous distribution of the genus 
may have been much greater formerly than at present and the 
species that occupied the intermediate areas may have since 
become extinct, or these isolated forms may owe their location 
to separate and fortuitous cases of long distance dispersal. If 
they do represent points in a former continuous distribution, 
judging from what we know of the other species and their in- 
creasing dissimilarity with increased distances from the supposed 
point of origin, we would expect these far-distant representatives 
to be the most aberrant and specialized members of the genus. 
As a matter of fact, the three species occurring in South America 
and the one in arctic America are very typical members of the 
genus and find their nearest relatives not in the species closest 
to them geographically, but in certain ones near the center of 
distribution. It is of interest also to note that the three South 
American plants differ considerably from one another and each 
finds its most nearly related form in North America. Due then 
to their dissimilarity from one another and their relationship 
to species near the center rather than on the periphery of the 
area of continuous distribution, it is thought likely that these 
four species occupy their present position because of some un- 
known agent of long-distance dispersal. The two species limited 
to Kentucky and Tennessee, however, do not seem to fall into 
the same category with the other isolated forms. Like them 
they are very dissimilar among themselves, but unlike them 
they are not similar to species rather near the hypothetical point 
of origin. Indeed, they are similar to no species now extant 
and they may well owe their isolation to the extinction of species 
that once occupied the intermediate areas. The occurrence of 
colonies of L. argentea in localities in Minnesota and Illinois is 
