E. V. Wulff —140— Historical Plant Geography 



"Since the tolerances of closely related species need not be closely comparable, it fol- 

 lows that the generic tolerance as regards single factors may be continuous or discontinu- 

 ous. If the former, the generic tolerance will include a complete range of values for a 

 particular factor, from generic minimum to generic maximum. If the latter, there will be 

 gaps in the range, these gaps representing factor values not included in any specific toler- 

 ance." 



In order to show just how the tolerance of a species is able, in case 

 of changes in external conditions, to induce its migration for the pur- 

 pose of finding suitable habitat conditions, Good takes as an example 

 a simple and purely hypothetical case of a species whose tolerance is 

 determined by one external factor only, temperature (actually, of 

 course, several factors are always involved). Good (p. i6i) presents 

 this example as follows : — 



"Let us imagine a species having a mean annual tolerance of io°-2o° C. . . . and let 

 us suppose that the area over which these temperature values prevail, the potential area of 

 the species, is continuous. Let us further imagine that the species has covered the whole 

 of this area so that the area of the species and that of the appropriate temperature condi- 

 tions are coterminous. 



"Every generation, if the species is herbaceous and monocarpic, and every reproduc- 

 tive season if the species is perennial, the individuals comprising it will produce dispersal 

 units (usually seeds or fruits) and these will tend to be disseminated in all directions from 

 the parent plants". 



In case these seeds germinate within the existing area of the species, 

 the plants arising therefrom will have all the necessary conditions for 

 normal growth and development. If, however, these seeds are carried 

 beyond the boundaries of the area of the species, i.e., beyond the range 

 of tolerance of the species to temperature conditions, then, if there do 

 not arise mutant forms with a different range of tolerance, the plants 

 produced from these seeds will perish or, at any rate, will not develop 

 normally. "This process", to quote Good (p. 162), "will continue at 

 reproductive intervals of time as long as the specific tolerance and dis- 

 tribution of temperature remain unchanged, a proportion of the units 

 at each reproduction failing to develop". 



"Now suppose", continues Good, "that a climatic change associated with general 

 lowering of temperature begins. Other things (such as topography) being equal, the tem- 

 perature area of io°-20° C. will move towards the equator. What will be the effect upon 

 the plants? On the northern edge of their area, as climatic movement begins, there will be 

 the equivalent of a contraction of potential area so that some of the dispersal units, not 

 only of the outermost plants but also of those slightly further in, will fall outside the neces- 

 sary conditions. Before very long the parents which were originally the outermost will be 

 themselves outside the potential area and will therefore perish. As the cUmatic movement 

 continues the belt of destruction in its wake will widen. 



"On the southern edge of the specific area the conditions will be exactly reversed. 

 After a time none of the dispersal units of the outermost individuals will any longer fall 

 outside the potential area, but within it, and will mature successfully, producing dispersal 

 units in their turn. These new individuals will at first disseminate themselves partly out- 

 side the area, but very soon, with the continuance of climatic change, their dispersal units, 

 too, will fall within the necessary conditions, and this process will be repeated in succeed- 

 ing generations [until, of course, the cUmatic movement ceases]. 



"The combined effect on the southern and northern edges of distribution, together with 

 similar but modified effects on the flanks, will in fact be such that correlation is main- 

 tained between climatic and specific area, and hence, since the former moves, so also does 

 the latter. But this movement will only result if tolerance remains unchanged while climate 

 alters". (Italics in original). 



