26 A MONTANE RAIN-FOREST. 



These considerations lead me to the surmise that the patches of Dan- 

 thonia on Sir John Peak and Mossman's Peak are relicts of a former 

 extensive alpine grassland formation which has been encroached upon 

 by the forest as the mountains have been worn down below the tree 

 limit. 



HABITAT DISTINCTIONS IN THE RAIN-FOREST. 



An examination of the forest formation which clothes the Blue 

 Mountain range reveals both vegetative and floristic differences in 

 its character in different localities. These differences are due (1) to 

 the climatic difference between the northern, or windward, and the 

 southern, or leeward, slopes of the range, which has its basis in differ- 

 ences in precipitation and the number of hours of fog and sunshine; 

 (2) to local differences due to the highly dissected erosion topography, 

 which have their basis in differences of atmospheric humidity and wind 

 action; (3) to the altitude, which has its basis partly in temperature 

 differences. The marked climatic difference between the northern 

 and southern slopes, due to the prevailing direction of the trade wind, 

 operates in a manner and direction such as to obscure any influence 

 which the direction of slope in relation to insolation might have in 

 differentiating the conditions for vegetation on the north and south 

 slopes of the range. The low latitude of Jamaica makes this a factor 

 which would not be operative in any case for more than a few months 

 in winter. In like manner the striking difference in conditions of 

 atmospheric humidity between the bottoms of ravines and the summits 

 of ridges tends to obscure any influence which differences in amount 

 of soil moisture might have in these habitats during the relatively dry 

 periods which occasionally supervene. In view of the excellent dis- 

 tribution of the rainfall I feel confident in stating that the fluctuating 

 amounts of soil moisture are a negligible factor in the distribution of 

 vegetation. During the very exceptional dry periods, such as that 

 which has been mentioned as occurring at New Haven Gap in April, 

 May, and June, 1892, the depression of soil-moisture content would 

 no doubt be sufficient to defoliate, if not to kill, the most hygrophilous 

 shrubs and herbaceous plants, particularly as such a rainless period 

 would be one of high percentage of insolation, high temperatures, and 

 low humidity. 



The differences in temperature which exist between sea-level and 

 4,500 feet are profoundly significant to vegetation. The Smaller differ- 

 ence which exists between the 4,500 feet (1,372 meters) level and the 

 summits of the three highest peaks is of no such importance, although 

 it appears to be responsible for the limiting of the vertical distribution 

 of many species. During the day the uniformity of moisture conditions 

 on the windward slopes from 4,500 to 7,400 feet (2,250 meters) tends 

 to offset the most important of the temperature influences, that is, on 



