50 DISTRIBUTION AND MOVEMENTS OF DESERT PLANTS. 
The number of plants on the Laboratory domain that are provided 
with hooks, spines, etc., is relatively large. At certain seasons of the 
year it is almost impossible to walk about on Tumamoc Hill without 
carrying on one’s clothing a collection of burs of Harpagonella, the siliques 
of Thelypodium lastophyllum, or the equally tenacious fruits of other 
genera. ‘The seeds of such plants are distributed in great numbers by 
domestic animals, and doubtless by others whose part in transportation 
is less easily observed. The part played by domestic animals is perhaps 
most conspicuously shown by the ubiquitous cocklebur, which, from 
Texas through New Mexico and Arizona to California, has crowded into 
roadsides, river-bottoms, and waste lands generally, and is everywhere 
seen with its prickly burs matting the tails of horses and cattle, and 
constantly carried by them to whatever fresh spots have not yet been 
invaded. <A striking instance of the interrelations of different animals 
with a single plant, now widely distributed through the southwest, is 
described by Tower (1906). Solanum rostratum, which grows as a weed 
around corrals and watering-places, has followed the two chief routes 
of travel from northern Mexico into the United States—the one into 
Texas, the other into New Mexico and Arizona—having been carried 
along these routes in early times, as now seems certain, by Spanish pack- 
trains. This is the favorite food-plant of Leptinotarsa intermedia, a spe- 
cies closely allied to the Colorado potato-beetle. Apparently the beetle 
and the Solanum extended their range together from Mexico early in 
the eighteenth century, and to the present time have advanced parz 
' passu, since it is found at the present day that as new routes of travel 
into irrigated arid lands are opened they move into them and together 
occupy lands where they had not hitherto been found. Thus incident- 
ally, in the Spanish conquest of Arizona and New Mexico, the animals 
of the pack-trains brought with them and effectually disseminated the 
seeds of a plant, which by other means, probably by herds of bison, was 
later carried farther north, where it has still been the food of Leptin- 
otarsa intermedia, and for a time of the economically more important 
and dreaded allied form, the potato-beetle. 
Water is perhaps to be classed with the agencies of dissemination 
which, in the region of the Laboratory domain, are of minor importance, 
but it is none the less an efficient factor. The heavy rains that gulley 
out the mountain sides, though relatively infrequent, carry multitudes 
of seeds from higher to lower levels, the tendency being to obscure zonal 
distribution, in so far as the action of entirely different factors makes 
this possible. In some parts of the desert region, however, it is probable 
that at the present time water is the most effective of all the agencies 
named. During the Salton Sea expedition of February, 1908, Dr. D. T. 
MacDougal and Dr. W. A. Cannon visited various points of the shore 
and the islands in the southern part of the sea, for the purpose of ascer- 
