RELATIONSHIPS OF METASPERMAE 585 



would, however, be proportional to the increase in the length 

 of the parallels, so that, in point of position by longitude, the 

 diiferences in plant groups, species and formations vary- 

 directly with the distances they are from each other. In this 

 case, thus generally stated, the differences in elevation, trend 

 of isotherms, direction of prevailing winds, etc. , which might 

 be found on the same parallel of latitude, are disregarded in 

 order to simplify the statement. Such differences would have 

 only a modifying, not a fundamental effect on the facts of 

 distribution. 



But if the traveler selected some meridian for the line of his 

 journey around the earth, the changes in the plant-inhabitants 

 as he passed from region to region would be greater in amount 

 and very much more conspicuous than in the former case 

 where a parallel of latitude was selected. In circling the 

 northern hemisphere one may, at a moderate degree of latitude, 

 pursue almost the entire journey in a coniferous forest over 

 the land areas, and in a region of distinctive fucoid and red 

 algae over the water areas of the journey. No such uniformity 

 of floral aspect would be maintained if a meridian be selected. 

 From the sphagnum and tundra region of the pole, one 

 would pass into coniferous forest, hardwood forest, evergreen 

 tropical forest, and into the deserts, savannahs and virgin 

 forest of the equatorial region. Then in inverse order the 

 same changing panorama with, however, an almost entirely 

 new series of forms would unfold itself as the traveler neared 

 the opposite pole. In the course of his journey he would find 

 that the greatest differences of all are those that exist between 

 the plant-inhabitants of the north temperate and south temper- 

 ate regions. The difference between the United States and the 

 Argentine Republic is far more conspicuous than the difference 

 between the United States and Siberia or Europe. And in like 

 manner the difference between Asia and Australia is greater 

 than that between Australia and the Cape of Good Hope or 

 Chile. 



In general, in either the northern or the southern hemis- 

 phere, in point of position by latitude, the differences in plant 

 groups, species or formations varies directly with the differ- 

 ences in humidity. As the equator is approached the average 

 annual precipitation progressively increases over most of the 

 surface of the earth. This is due either directly or indirectly 

 to the progressive increase of temperature. While this suffices 

 to explain, the differences between two more or less distant 



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