148 THE AMERICAN BOTANIST 



range as wide as its family, that it is probably the parent of 

 the rest of the family. The smaller the area over which a 

 species is distributed the younger the species and, if it has not 

 had time to spread far and wide, it is naturally endemic. In 

 general the world over, the most widespread species are the 

 oldest in comparison with the other species in the group to 

 which they belong. The methods which plants have adopted 

 for the spread of the species, however, has something to do 

 with the case. The dandelion would naturally spread faster 

 than the hickory-nut or the osage orange. The hypothesis, 

 however, will be of use if even in a general way it indicates 

 the differences in age of different plant groups, since this will 

 throw more light upon their evolution. 



Soil and the Plant Covering. — Exactly how much the 

 character of the vegetation in a given locality depends upon 

 the soil in which it grows is rarely realized by the casual ob- 

 server. We know, from repeated experience, that like regions 

 are pretty certain to contain the same kind of plants, but we 

 fail to appreciate the fact that this is due to the soil. The 

 difference in vegetation is especially noticeable when two very 

 different soils happen to adjoin one another, as in the sand 

 dune region about Lake Michigan. In the drift soils border- 

 ing the sand dunes, the common spring flowers are hepatica, 

 bloodroot, adder's-tongue, trillium, phlox, Dutchman's 

 breeches, squirrel corn, and the common blue violet, while on 

 the sand dunes none of these occur. In their places we find the 

 birdfoot violet, rock cress, bluet, huckleberry, sassafras, lupine, 

 and large puccoon. None of these are to be found in large 

 areas of the drift soils. In the rich black soils of swampy 

 tracts in either region, however, there is much less difference 

 between the plants because such soils are essentially alike. In 

 the swamps we find cowslips, skunk's cabbage, painted cup, 

 swamp saxifrage, white violet, squaw weed and similar 

 species. 



