266 



for the invasion of one province by another; that is to say, the dom- 

 inant species of the invading association are the pioneer species in the 

 invasion. Many other general principles concerning succession might 

 be given, but as Adams has summed up "Some Principles of Suc- 

 cession" tlie reader is referred to "An Ecological Survey of Isle 

 Royal, Lake Superior," pages 146 to 149, (1909) for their state- 

 ment. A relic species exists in a given association because it occupies 

 ground which as yet is not tenantable by any of the species of the 

 succeeding association, rather than because the succeeding association 

 can not displace the relic. An invader occupies more nearly its opti- 

 mum habitat, but the relic lives where the other plants seem not to 

 be able to develop. The disappearance of the relic usually takes 

 place with the death of the individuals, whereupon the bit of ground 

 which it occupied may be taken up almost immediately; or again 

 — and many instances are at hand — the spot may remain bare for 

 some time to come. Some relics modify the structure of their vege- 

 tative parts and continue for a long time after the invasion has been 

 completed. Junipers and Rhus canadensis illinoensis (sumac) are 

 two very good examples of this class of plants. 



The naming of the associations has been approached from many 

 different view points, but the most natural course seems to be to use 

 the name of one or more predominating species, and, accordingly, 

 that method is adopted in this article. In cases where another inves- 

 tigator has found associations clearly the same as those of the Beach 

 area, the name that he used will be given first consideration, priority 

 being regarded in so far as the fitness of the subject will permit. 



The Lower and Middle Beach Associations 



As the waters of Lake Michigan receded, a sand beach was ex- 

 posed. This furnishes the starting point of a genetic series of asso- 

 ciations which is known as the beach succession. Bare sand and the 

 water of the streams and lakes are the two initial points of primary 

 successions in this area. 



THE CHLAMYDOMONAS ASSOCIATION 



The classification of lake beaches has usually been founded upon 

 a physiographic basis, in which the features distinguished are lower, 

 middle, and upper beaches. The "lower beach" of Lake Michigan has 

 been defined by Cowles (1899:113) as "that zone which is situated 

 between the water level and the line reached by the waves of common 

 summer storms." He gives an alternate definition on page 114: "It 



