Chapter V 



—67- 



Types of Areas 



with true vicarism, is a phenomenon simulating the latter but of an 

 entirely different nature. 



The problem as to the origin of vicarious species and areas has con- 

 stituted one of the most difficult problems of plant geography, inade- 

 quately solved until quite recently. As early, however, as 1869 Ker- 

 NER pointed out that the present area of a species cannot always be 

 regarded as the Hmit of its possible distribution; in many cases its 

 present boundaries represent only those which the species at the given 

 moment has reached in its distribution, the explanation of its absence 

 on the other side of these boundaries being simply that it has not yet 

 had time to go beyond them. But, at the same time, it is perfectly 

 clear that in many cases habitat conditions — climatic and edaphic— 



Fig. 8. — Area of distribution of Draba luteola in the Rocky Mts. (main area), and of its 

 vicarious variety, var. minganensis (the two small black spots) near the Gulf of St. Lawrence, 

 the latter having arisen as a result of isolation. (After Victorin). 



constitute insuperable obstacles for the species in its distribution. 

 Such conditions may be, for example: decrease in the sum total of 

 heat during the summer months, shortening of the vegetative period 

 by early spring or fall frosts, summer drought, decrease in the annual 

 precipitation, change in the distribution of precipitation through the 

 seasons of the year, replacement of some soils by others markedly 

 differing in chemical or physical properties, etc. In addition, there is 

 no doubt that on the periphery of an area, toward the margins of the 

 range of a species, the competition of other species may also constitute 

 a barrier to further distribution. 



These circumstances compel us to conclude that the area of any 

 species may be divided into two parts. In one the species finds itself 



