265 



matic association which, if conditions were ideal and sufficient time 

 were allowed, would occupy the whole of the given area. The series 

 of associations which succeeded one another from bare ground to the 

 climatic type is known as a genetic series. It is not necessary, how- 

 ever, that successions in a given area should proceed according to 

 the normal genetic series. Mishaps of various kinds are continually 

 occurring to prevent this. Successions are recognized primarily by 

 the presence of pioneer or relic species within a given association. A 

 pioneer species, as its name implies, is a species of a given association 

 that can invade a genetically lower association, and a relic species 

 is a species of a preceding association which remains after a success- 

 ful invasion, thereby giving a clue to the situation. From this it 

 follows that a complete association — if one may be allowed to use 

 that term in this connection — consists of dominant species, secondary 

 species, whose varying seasonal dominance produces seasonal aspects, 

 invaders or pioneer species of a succeeding association, relics of a 

 former association, together with such ubiquitous species, which seem 

 to have little or no restriction placed upon their distribution, as may 

 occur there.* 



Successions form the most satisfactory approach to the ecological 

 study of a region, and for this reason it may be well to give the sub- 

 ject brief consideration. As mentioned above, successions are often 

 easily recognized in an association by the presence of pioneer or relic 

 species. When associations within one formation are concerned, suc- 

 cession usually takes place by the invasion of the secondary species of 

 the invading association, and the succession may be said to be com- 

 pleted when the dominant species have made their appearance. In 

 the case of the invasion of an association of one fonnation into an 

 area occupied by an association of another formation, invasion is ef- 

 fected by the dominant species, with the subsequent appearance of 

 the secondary species. As one w^ould naturally expect, invasion of 

 one formation into another takes place through the pioneer associa- 

 tion, which is characterized by a paucity of species, relatively speak- 

 ing, and, consequently, in such an area the vegetation consists of the 

 dominant species of the invading association with such of the species 

 of the invaded one as can live under the new conditions. These 

 secondary species are existing there as relics, yet they comprise vir- 

 tually all of the secondary vegetation. This same principle holds also 



*For further discussion of the association consult: 

 Clements, F. E. Research Methods in Ecology. 1905. 



CowLES, H. C. The Causes of Vegetative Cycles. Bot. Gaz. 51 :i6i-i83, Mar., 

 igir. 



