SUMMARY 



471 



zone merges more or less gradually, as the ground rises 

 and becomes drier, with the surrounding vegetation type, 

 forest (Fig. 138) or meadow, for example. Within each 

 zone there are usually more than one species of plant; 

 and moreover, the species found in each vary according 

 to the particular region in which the pond occurs. 



Summary. — Species of plants and animals have come 

 into existence through evolution in all parts of the 

 habitable earth. They have in the course of time spread 



Fig. 138. — Bog Vegetation of the Northeastern Coniferous forest 

 region. Surrounding open water is a zone of an evergreen shrub 

 of the heath family; in the background is the eastern larch or 

 tamarack. (H. A. Gleason.) 



away from the places of origin in whatever directions 

 they encountered suitable conditions. They have not 

 reached all the places suitable to them because they have 

 usually encountered barriers of one sort or other that 

 check or stop them altogether. What is a barrier to one 

 organism may be the means of travel to another or at 

 least present little difficulty in being passed. Because 

 all organisms do not find the same conditions equally 

 suitable for their habitation, there arises a segregation of 

 those with like adaptabilities in the same or similar 

 regions. Since those plants adapted to similar habitats 

 generally possess similar appearance, there rises the 



