RELATION OF CONDITIONS TO HABITAT DISTINCTIONS. 41 



is playing through its usual range in the vicinity of Cinchona (see plate 

 22). That other species of the Hymenophyllacese have acquired semi- 

 xerophUous characteristics which enable them to persist in the mid- 

 levels of the forest in company with Caraguata (see plate 22) and to 

 endure the same conditions to which it is liable, is one of the most 

 striking features of the rain-forest. 



THE RELATION OF PHYSICAL CONDITIONS TO HABITAT 

 DISTINCTIONS IN THE RAIN-FOREST. 



During my visit to the Blue Mountains in the winter of 1905-06 I 

 carried on instrumentation designed to give some evidence as to the 

 degree and manner in which the climatic conditions within the rain- 

 forest depart from the normal conditions of the open slopes at Cinchona 

 on which the longer series of data was secured which have already been 

 presented; and also to determine what some of the differences of con- 

 ditions are that may be responsible for the distinctions in the vegeta- 

 tion of the habitats that have been described. 



It requires but a casual visit to the region to realize that the most 

 salient characteristics of the vegetation are determined by the high 

 rainfall — ^unbroken by a pronounced dry season — together with the 

 high percentage of cloudiness and fog, with all the subsidiary condi- 

 ditions of moist soil, moist atmosphere, small percentage of insolation, 

 wetness of fohage and the like, which follow in their train. Further- 

 more the moisture conditions are the most important set of differential 

 factors in determining the diverseness of the several habitats. 



Rainfall readings are almost meaningless for a region in which, as 

 here, ten showers of two hours' duration each may give only a total 

 fall of 1 to 2 inches, whereas on another day a single fall of two hours' 

 duration may give the same amount, with a totally incommensurate 

 effect on the other moisture conditions and on the vegetation. Further- 

 more, a light rain followed by several days of continuous fog will have 

 a very different significance from a heavier fall followed by two or three 

 hours of insolation. The irregularity of the rainfall (see p. 15) together 

 with the fact that the moistness of the atmosphere, the wetness of the 

 foliage, and to an extent even the moistness of the soil, are due as much 

 to fog as to actual precipitation of drops large enough to be called rain, 

 gives the rainfall figures only the most general bearing on the conditions 

 present. So well distributed is the rainfall, so low the evaporating 

 power of the air, and so unbroken the vegetational covering, that the 

 state of moistness of the soil is a factor which can be safely neglected 

 throughout periods of normal weather. I have already called atten- 

 tion to the occasional periods of very light rainfall, during which it is 

 possible for the soil moisture of the ridges and peaks to fall to an extent 

 that would make this factor one of importance. I had an opportunity 



