178 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



the precipitation amounted to 13.5 inches. From 15 to 20 miles inland, at Merida, and 

 at Motul and Temax which lie farther east, the precipitation for the same year was 35.7, 

 37.6, and 34.8 inches, respectively. Still farther inland, at places varying from 30 to 90 

 miles from the coast, the figures are as follows: Izamal, 40 miles due east of Merida, 49.2 

 inches; Espita, nearly as much again to the east, 48.7 inches; Tekax, 50 miles south-south- 

 east of Merida, 53.3 inches; and Peto, about 30 miles .southeast of Tekax but not so much 

 among the hills, 47.7 inches. Finally, to the east and south of the places already mentioned 

 we find an area of still larger rainfall, exemplified by ^'alladoHd, which lies 100 miles east- 

 southeast of Merida and about 50 miles from the Caribbean Sea. It had a rainfall of 

 66.8 inches in 1911. Southward beyond this point, to judge from the vegetation, the 

 precipitation becomes still greater. The cause of the variation in rainfall is twofold. In 

 the first place, the presence of hills in the south and southwest on the one hand, and the 

 abundance of easterly oceanic winds on the east coast, give those regions much rain. In 

 the second place, we are here near the edge of the area reached by the zone of subeciuatorial 

 rains. Hence the amount of these rains increases rapidly toward the south. 



With such marked changes in the amount of rainfall from place to place, it is evident 

 that the vegetation must vary greatly, and that this fact in turn must profoundly affect 

 the conditions of human hfe. The botanical works commonly emphasize the distinction 

 between tropical l)ush, tropical jungle, and tropical forest. Nevertheless, in the mind of 

 the average geographer, if I may judge from my own experience, and still more in the 

 mind of the layman, the distinction often lacks sharpness. There is a still greater lack of 

 appreciation of the significance of these three types in their effect on man. In Yucatan 

 bush, forest, and jungle lie so close together that they can readily be compared. In 

 the center of Yucatan lies a long narrow lake called Kichankanab, one of several which 

 occupy hollows in the limestone of the southerly, more hilly portion of the peninsula. 

 It is located about 100 miles east of Campeche, 100 west of the Caribbean Sea, and 100 

 south of the northern shore of the peninsula. If lines be drawn northeastward and north- 

 westward from the lake to the corners of the peninsula they will include approximately the 

 entire area of the Mexican administrative province of Yucatan, which comprises only 

 about one-fifth of the whole peninsula. This small fifth of the countr,y, together with a 

 strip of the west coast reaching down toward Campeche, comprises the bush-covered 

 portion, while the rest is covered with jungle or genuine forest. The western boundary of 

 the bush area is nearly coincident with the small range of hills already mentioned as the 

 most noticeable feature of the rehef. The eastern boundary appears to be less distinct, 

 although I have not seen it and can not speak with assurance. Where bush prevails 

 the rainfall seems not to exceed 30 or 40 inches, while in the forested area it rises far 

 higher. How great it is we do not know, for \'alladohd with nearly 67 inches in 1911 is 

 the only station whose figures are obtainable, and it lies on the relatively dry edge of the 

 forest, not in its moist interior. 



The distinction between bush, jungle, and forest is simple. Large trees demand that the 

 soil in which they stand shall not be dry for any great length of time during the growing 

 season. Inasmuch as the growing season may last the entire year in the tropics, large 

 trees will not flourish in such a way as to form dense forests unless abundant rain falls at 

 most seasons, although they may grow sporadically here and there. Smaller, more drought- 

 resistant species, however, as well as bushes, are much less exacting in their demands for 

 moisture. Some of them will grow almost anj^^^here provided that the ground is well 

 moistened for 2 or 3 months during some portion of the year and there is sufficient warmth. 

 In regions hke Progreso, on the north coast, where the rainfall is only 10 to 15 inches, 

 concentrated largely in the sununer, the long dry period of winter prevents the growth of 

 anything except small bushes 6 to 8 feet high; these, however, thrive in abundance, so that 

 the countrj^ is well covered with vegetation and is everjTvhere bright green in summer. 



