CH. XIV] VEGETATION FORMS 559 



Among higher forms, however, this is less the case, again as 

 among men of civilized races, where members of the same 

 family often scatter into the most diverse occupations, 

 following each the opportunity indicated by his own tastes 

 or talents. Thus the ghyjogenetic and the ecological groups, 

 while partly coincident, are largely different. 



2. PLANT HABITATS AND VEGETATION FORMS 



The surface of the earth presents a variety of situations in 

 which plants can live. Such situations are called HABITATS. 

 Observation shows that each habitat is occupied by certain 

 forms of vegetation which are better fitted than other kinds 

 to the conditions there prevailing. These are called VEGETA^ 

 JTION FORMS, or GROWTH _.EfmMs (though they were better 

 called HABirJV>RMa) ; and they constitute the dominant vege- 

 tation of that habitat. Thus the condensed, thick-epidermed 

 stem succulents, familiar in the Cacti, are a dominant vegeta- 

 tion form of some deserts, while at the opposite extreme the 

 diffuse, thin-tissued^ Waterweeds are a dominant form of 

 quiet waters. Thus these vegetation forms constitute the 

 units of the ecological classification of the vegetation of the 

 earth, just as species are the units of the phylogenetic classi- 

 fication of its flora (page 392) . Vegetation forms and species, 

 however, are not coincident, because all species possess 

 features, hereditary, structural, or mutational (pages 

 12-13), not connected with their fitness to the habitat, and 

 therefore not a part of their vegetation form. Moreover, 

 several or many species, and from widely separated families, 

 can belong to the same vegetation form. Thus the Water- 

 weeds, above mentioned, contain members of several fami- 

 lies; and there are species of Euphorbia, living in deserts, 

 which are so like some Cacti that only a botanist can tell 

 them apart, though the families Cactacese and Euphor- 

 biacese are widely separated phylogenetically. Vegetation 

 forms, accordingly, are far less numerous than species, and 

 their actual number depends upon the extent to which we 



