Overlapping Habitats. 



77 



which the rainfall is very low, and the mesophytic period con- 

 stantly brief, altogether. 



Now the two slopes, one facing the north and the other the 

 south, are analogous to two deserts, with a long and with a 

 short mesophytic period. We may assume that they receive 

 the same rainfall, as indeed they do. Under the conditions of 

 insolation, however, the same rainfall does not produce meso- 

 phytic periods of the same length. The higher temperature 

 and consequent evaporation rate on the south slope reduces 

 more quickly the superficial soil moisture, while at the same 

 time the isobar of vapor tension is nearer to the surface of the 

 soil. The relatively thin blanket of vapor thus produced does 

 not serve to effectually protect the young seedling, which on the 

 the north slope finds far more congenial conditions sustained 

 for a longer period. This view is supported by the character 

 of the other plants which occur on the two slopes. On the north 

 slope are to be found algae, mosses, liverworts, the prothallia 

 of ferns and selaginallas (2 species) and their delicate young 

 sporophytes in abundance, which are absent almost entirely 

 from the south slope. The vegetation of the south slope, on 

 the other hand, is made up very largely of succulents, Agave 

 lecheguilla, Hectia sp., Jatropha, Candelilla (Euphorbia anti- 

 syphilitica), large cacti (Biznaga burr a and color ada) and the like, 

 plants which may occur, but in much less number, on the oppo- 

 site slope, where palma samandoca, low shrubs and ferns are 

 dominant. 



On the adjacent footslopes the conditions are intermediate 

 between those of the north and south slopes, being somewhat 

 less rigorous than on the south slope and somewhat more so 

 than on the north slope. As a consequence, we find that, 

 while there is here an intermingling of types, among which are 

 ocotillo and palma samandoca, others (some succulents of the 

 south slope and the ferns of the north slope) are absent. As- 

 suming the adequacy of our explanation, the footslope is a desert 

 where the conditions are so balanced as to bring together forms 

 which may be separated by a small departure toward greater 

 or less severity. As a corollary we can see that a slight change 



