THE POOTUILL VEGHTATION 109 



formity in the percentages of soil moisture. It is obvious also 

 that the water supply in soils is depleted by absorption and 

 transpiration, which may in large measure account for the dry- 

 ness of the soil on the more heavily wooded slope. 



The yellow pine seldom favors the steeper north slopes, 

 even where space is available, except upon outjutting promon- 

 tories or elevations, which seems to indicate certain relations to 

 light or temperature. Experimental operations show that the 

 yellow pine grows more rapidly, on the whole, exposed to full 

 light at Missoula than it does under half shade, while the reverse 

 is true of the Douglas spruce. 



It has been suggested above that the study of distributional 

 problems should be approached from the standpoint of the nature 

 of the species, rather than from the standpoint of the external 

 factors. Ecological literature has usually been concerned more 

 with the effect of climatic and other factors upon the vegeta- 

 tion as a whole on a given area and for the most part has neg- 

 lected the critical examination of the specific qualities which 

 identify the important species. From this point of view phy- 

 sical data is none the less necessary, but each species is a complex 

 entity which reacts to its environment in its own way. Each has 

 its individuality and the study of that individuality, the specific 

 reactions to light, temperature, moisture, etc., efficiency in seed 

 bearing, periods of fruiting, immunities and the like, in short 

 the- sum total of its resistance and response, must throw light 

 upon the nature of the plant and the conditions of its distribu- 

 tion. Every species is a complex of positive and negative quali- 

 ties, the positive qualities those that are advantageous, the nega- 

 tive those that are disadvantageous. The balance of these 

 qualities may show an excess for or against the species, and 

 this balance will be found to be proportional as a rule 

 to its geographic range and abundance. The negative 

 qualities are limiting factors in its distribution, the positive 

 make for dispersal. The more stringent the limitations the 

 more circumscribed the distribution locally and generally, al- 

 though in the latter aspect the limits of geographic range are 

 not clue to the lack of similar and suitable conditions elsewhere, 

 so much as to the more effective action of intervening barriers. 

 These conclusions, it is thought, will be supported by reference 



