Some general idea of the influences which have determined the distribution of 

 species will perhaps be gained from the following brief extracts from the botanical 

 report : 



"The causes which have controlled the distribution of plants over any portion 

 of the earth's surface are intimately connected with, and often seem to center in, 

 its geological and physical structure and its climate; or more immediately in the 

 chemical composition and mechanical structure of its soil ; the prevailing tempera- 

 ture ; and the average precipitation of moisture. The radiation of species from 

 their place or places of origin — which in the minds of some naturalists has had 

 such controlling influence in giving diversity to the floras of different continents, 

 or even to distinct geographical districts of the same continent — has had no ap- 

 preciable bearing on the distribution of plants within the limited area of the State 

 of Ohio ; an area which forms but a part of another and far greater one over 

 which the vegetation has a common character, and in which the distribution of 

 plants has been determined mainly, if hot only, by local causes. As a single 

 block, however, may be an essential part of a great and complicated structure, so 

 the florula of Ohio will doubtless some day be shown to be an indispensable ele- 

 ment in the great systematic whole of the flora of America, which future botanists, 

 wiser than we, will be able to portray not only in all its internal symmetry, but in 

 its relations to the present vegetation of the entire globe ; and perhaps even to 

 that which existed on our planet during the different geological periods. With all 

 that, however, we have at present nothing to do. I do not now know cf a single 

 plant which is peculiar to the surface of our State, though it is possible there are 

 a few such. The botanical importance of the flora of Ohio consists exclusively, 

 then, in the grouping of its elements whhin this area, and the relations of these 

 groups to the floras of adjacent and larger surfaces. That greatest diversifier of 

 vegetation in districts of limited extent, altitude, has played no part in the distri- 

 bution of our plants, as there are no points within the State which rise five hun- 

 dred feet above the average plane, and none that are one thousand feet above the 

 lowest. 



"Differences of temperature, depending on differences of latitude, have pro- 

 duced more appreciable results, and it will be seen by reference to the accompa- 

 nying catalogue that the number of species restricted to the northern or southern 

 portions of the State is considerable, though as compared with those whose range 

 is general or restricted by other causes, it is small." 



" The influences which have affected the range of plants east and west are, as 

 we shall see, quite as potent as those which act along the meridians of longitude. 

 These causes center in the differences of physical and geological structure pre- 



