Western frontiersmen and others who live in the land of the pack rat relate stories innumerable of the absurd but industrious doings of these eccentric creatures. The ways of the pack rat are so erratic that I find it impossible to figure out by any rules known to me the workings of their minds. Strange to say, they are not fiends and devils of malice and destruction like the brown rat of civilization, and on the whole it seems that the destruction of valuable property is not by any means a part of their plan. They have a passion for moving things. Their vagaries seem to be due chiefly to caprice, and an overwhelming desire to keep exceedingly busy. I think that the animal psychologists have lost much by so completely ignoring these brain-busy animals, and I hope that in the future they will receive the attention they deserve. Why experiment with stupid and nerveless white rats when pack rats are so cheap? It was in the wonderland that on the map is labeled "Arizona" that I met some astonishing evidences of the defensive reasoning power of the pack rat. In the Sonoran Desert, where for arid reasons the clumps of creosote bushes and salt bushes stand from four to six feet apart, the bare level ground between clumps affords smooth and easy hunting-grounds for coyotes, foxes and badgers, saying nothing of the hawks and owls. Now, a burrow in sandy ground is often a poor fortress; and the dropping spine-clad joints of the tree choyas long ago suggested better defenses. In many places we saw the entrance of pack rat burrows defended by two bushels of spiny choya joints and sticks arranged in a compact mound-like mass. In view of the virtue in those deadly spines, any predatory mammal or bird would hesitate long before tackling a bushel of solid joints to dig through it to the mouth of a burrow. Did those little animals collect and place those joints because of their defensive stickers,--with deliberate forethought and intention? Let us see. In the grounds of the Desert Botanical Laboratory, in November 1907, we found the answer to this question, so plainly spread before us that even the dullest man can not ignore it, nor the most skeptical dispute it. We found some pack rat runways and burrow entrances so elaborately laid out and so well defended by choya joints that we may well call the ensemble a fortress. On the spot I made a very good map of it, which is presented on page 164. [Footnote: From "Camp-Fires on Desert and Lava" (Scribner's) page 304.] The animal that made it was the White-Throated Pack Rat (_Neotoma albigula_). The fortress consisted of several burrow entrances, the roads leading to which were defended by carefully constructed barriers of cactus joints full of spines.