1921] 
PAYSON—MONOGRAPH OF THE GENUS LESQUERELLA 129 
for the three sections is found in the partiality of primitive mem- 
bers of each for soils rich in lime. Similarly, certain widely 
isolated species are here united to the genus by an additional 
character. 
Jordan's Law of Isolation.—It has been found through the 
study of geographical distribution of Lesquerella that in general 
Jordan's law of distribution holds good. This law is stated in 
the following terms: ‘‘Given any species in any region, the near- 
est related species is not likely to be found in the same region 
nor in a remote region, but in a neighboring district separated 
from the first by a barrier of some sort."' In Texas, species 
that seem not to differ greatly from one another are found in 
the same region, but practically without exception the species 
most nearly related is found in an adjacent region. As a rule, 
the ranges of related species overlap but little and this joint 
occupation may be apparent rather than real. That the species 
are separated by a barrier is not always demonstrable. It must 
be remembered, however, that our knowledge as to what con- 
stitutes a barrier is exceedingly limited. A change from cal- 
careous to non-calcareous soil, or a slight difference in water 
content might form an effective barrier. 
Summary.—Lesquerella seems to have arisen in the central 
Texas region, and from here the three sections and at least the 
auriculata, Engelmannii, argyraea, recurvata, and gracilis groups 
appear to have diverged. Since most of these various groups 
have reached a somewhat equivalent degree of specialization 
and have migrated long distances, it seems probable that they 
had their origin at about the same period. The Texas species 
at present appear to be in a rather static condition since the 
species are easily defined, but in New Mexico and Utah occur 
recent plexes of developraent to judge from the variability and 
number of the closely related species. From the Utah plexus 
it is believed that the genus Physaria has arisen. The primitive 
representatives of Lesquerella were probably partial to calcareous 
habitats. Since Jordan’s law of isolation seems to be supported 
by the present study, it will be necessary to take into account 
the characteristic separation of specific ranges in any inquiry 
IUDAS D. S. The origin of species through isolation. Science N. S. 22: 545. 
