ECOLOGY — INTERPRETATION OF ENVIRONMENT 321 



one plant society must necessarily be supplanted by another, 

 though the one passes into the other by imperceptible grada- 

 tions. This implies that environmental influences are cumula- 

 tive, that is — a plant society is not a product of present 

 conditions alone, but the past is involved as well. 



Great emphasis is placed upon border lines or zones of ten- 

 sion, for here, rather than at the centre of the society, one 

 can interpret the changes that are taking place. The various 

 plant societies pass in a series of successive types, from the 

 original hydrophytic condition through the xerophytic, to a 

 final mesophytic forest, the latter being regarded as a climax, 

 or culminating type. The trees making up this kind of forest 

 are often able to propagate from seeds within their own shade. 

 Sometimes, however, this order is not the one followed, as 

 retrogressive steps away from the mesophytic condition toward 

 the hydrophytic, or xerophytic, phase occur. Moreover, 

 "Changes may take place in plant societies more rapidly than 

 in the topography." On this assumption Cowles maintains 

 that in this region, as well as in the extensive area of the United 

 States east of the Mississippi, such forest trees as the beech 

 and maple have better chances ultimately in the contest for 

 space and time. 



In this case, societies of the small field daisy or thistle, which 

 are higher in the scale of evolution, will give way to tree forests. 

 Here we are reminded of Clodd's ^ remarks*about the daisy, and 

 he might have added the thistle: "Its position among plants 

 corresponds to man's position among animals. As man, in vir- 

 tue of being the most complex and highly specialized, is at their 

 head, albeit many exceed him in bulk and strength, so is the daisy 

 with its allies, for like reasons, above the giants of the forest." 



Clements, in the work previously cited, remarks in regard 

 to the dependencies and habitat of animals: "Vegetation indeed, 

 as the source of food and protection, plays a more obvious, if 

 not more important part. This is especially true of anthophi- 

 lous insects, that is, those that visit flowers, but it also holds 

 for all herbivorous animals and through them for carnivorous 

 ones. The animal ecology of a particular region can only be 

 properly investigated after the habitats and plant formations 

 1 "Primer of Evolution," 1895, pp. 55, 56. 



