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 
differences 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 tu 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, ata 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 suchuniformity 
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 
