instead, seems to require the recognition of additional abstract 

 types, separated from one another by arbitrary criteria. 2°°- ^^^ 

 Comparison may be made to Mendelian populations which are 

 recognized although they are not necessarily discrete units, and 

 often no sharp demarcation can be made between them. Like- 

 wise, youth and old age are recognized but they cannot be 

 sharply segregated. 



In a region where topographic changes are frequent, as in many 

 hilly and mountainous sections, the vegetation at first glance ap- 

 pears chaotic — the heterogeneity seems to be without "rhyme or 

 reason." The problem of individualizing and circumscribing 

 groupings in the complex mosaic of vegetation is an old one in 

 phytosociology. However, close inspection soon reveals that cer- 

 tain species are grouped together, although the proportions may 

 differ in different sites. The groups may be segregated by sharp 

 vegetational boundaries (ecotones) that may coincide with 

 abrupt changes in physical conditions in the soil such as texture, 

 structure, moisture content, or chemical constituents. ^^^^^ Other 

 sharply delimited stands may be the result of fire, cultivation, 

 animal activity, wind, or salt spray. Even in bogs sharp segrega- 

 tion is common,^ 2 but mosaic transitions may also occur. ^^^ 



Where changes in environmental conditions are gradual the 

 vegetation alters continuously from one kind of community to an- 

 other. But even under such conditions, discontinuities are often 

 present because one species may be the chief dominant in one part 

 of the gradient, but in another part, where the critical point of 

 tolerance has been reached, another kind suddenly takes its 

 place; however, there may be a narrow sector of the gradient 

 where both dominants are in competition.^^ In many areas both 

 abrupt and gradual changes between communities are present. 

 In an extensive study of forest succession in Algonkuin Park, 

 Canada, Martin^'^^ concluded that because of the great diversity 

 in the forests it is unfruitful to debate whether forest succession is 

 a continuum or a series of distinct communities. In some circum- 

 stances it is a continuum, in others a series of communities; or it 

 may be a continuum in one period, a distinct community in an- 

 other. In most cases elements of both are present in varying 

 proportions. 



Giroimpixmg of Species • 69 



