ACROSS PAPAGUKRIA. 



125 



A number of rodents fortify their burrows with joints 

 of the choUa which are dragged into such position around the 

 entrances as to allow a crooked passage for the occupants, 

 but w^hich would discourage the marauding coyote, cat or fox. 



The spines and glochids of the cacti do undoubtedly 

 lessen the ravages of grazing animals to some extent, but 

 no ground is afforded for the conclusion that the armature is 

 a direct and purposeful response to the injuries inflicted by 

 animals. The structures in question seem to have been in- 

 duced by aridity, and in a dozen species the evolutionary 



FIG. 1— Chollas (Opuntia fulgida and O. mamillata) in the vicinity of 



Tucson. 



development has been carried still farther and the spines are 

 reduced to the merest rudiments. Singularly enough, the 

 two poorly armed species, occurring in the vicinity of Tucson, 

 are not seriously molested by animals. It is also to be noted 

 that while the cacti seem to be especially suited for arid con- 

 ditions, yet many species occur in moist tropical forests. 



Two massive forms of melon cacti or bisnagas, {Ecliino- 

 ( (ictus zvizliceii'i! and E. citiuryi) were encountered by our 



