402 Annals of the Carnegie Museum. 



to the northeastern conifer forest and ends in the black oak forest with 

 more southern relationships. Fire by removing the organic matter 

 brings the habitat on Presque Isle back to practically sand-plain con- ' 

 ditions, and so invites invasion by species from the northeastern coni- 

 fer forest, but with the accumulation of humus in the soil and the 

 occupation of the habitat by sheltering vegetation more uniform con- 

 ditions of moisture and temperature obtain, and there is a gradual 

 reversion on the part of the vegetation to the deciduous forest 

 species. 



In considering the invasion of species or associations of species from 

 the one or the other forest center it must be remembered that in the 

 initial stages of the successions on Presque Isle the vegetation is more 

 or less " open," there being considerable unoccupied territory between 

 the individual plants of the formation, competition being therefore 

 reduced to the minimum. The beach-sand plain successions and the 

 dune successions do not really approach "closed" conditions until 

 the advent of the Pniniis serotina or the Pinus strobus forests, while 

 the lagoon succession remains more or less open until about *' Stage 

 E." 



The comparative rapidity with which the land form of Presque Isle 

 has been extended to the north and east makes necessary upon the 

 part of the plants a corresponding ability to migrate. The dissem- 

 inules of the plants must in some manner be distributed to a consider- 

 able distance from the parent plant, especially in the case of trees, 

 where a number of years must elapse before the attainment of a seed- 

 bearing age. 



Of the numerous agents of dissemination three are prominent among 

 the species of the initial stages of the successions occupying the newly- 

 formed land towards the northeastern extremity of the peninsula: (rtt) 

 water, (^b') wind, and (r) animals, mainly birds. Of these agents the 

 first, of course, prevails in the beach formations and in the fringing 

 Ammopliila dune, although indirectly wind has an important part in 

 this also. 



In the sand-plain and in the earlier stages of the lagoon succession 

 wind is evidently the most important agent of dissemination, although 

 for the initial invasion of a species around a lagoon birds are proba- 

 bly the all-important factor. The disseminules oi Jimcus, Eleocharis, 

 Cyperus, etc., are known to be carried considerable distances adhering 

 to the plumage or sticking in the mud on the feet of water-birds of 



