586 METASPERMAE OF THE MINNESOTA VALLEY. 
points on a meridian in either the northern or southern hemis- 
phere it does not explain the fact of the greatest average dif- 
ference that exists between north temperate and south temper- 
ate regions. To form an explanation of this, further examina- 
tion is necessary. 
The equatorial or tropical regions of the earth, since in 
point of humidity and temperature they present the most fav- 
orable conditions for plant-growth, are crowded with a luxuri- 
ant vegetation. This crowding of the favorable region might 
be likened to the congestion by men of a rich gold-field where 
the opportunity of acquiring wealth is most favorable. Under 
such conditions the struggle for existence becomes most bitter 
and, as in countries overcrowded with humanity, an escape is 
made, when practicable, by emigration. The equatorial region, 
then, is a perennial fountain-head from which there is a con- 
stant stream of emigration into northern and southern lati- 
tudes. With such migration there must, under the stress of 
natural selection, originate and develop modifications in the 
migrating forms, which in course of time arise to specific 
rank. What these modifications may be in any particular case 
depends upon the complicated intermingling of the various 
particular conditions of climate, nutrition and competition. 
Further it happens that cyclical changes in the mean tempera: 
ture of polar or subpolar regions have, at different times, ini- 
tiated glacial epochs of longer or shorter duration. In the 
northern hemisphere the glaciers have extended south in Asia 
to the Himalaya mountains and in North America at least 
to latitude 39°, in Missouri. The effect of secular ice in- 
vasions upon a highly developed plant-population, could not 
be other than disastrous. Before the advancing glacier there 
must have been, among plants as among animals, a stern race 
for lower latitude and more congenial temperature. In this 
way periodic returns to the equatorial belt have been chara- 
acteristic, in a general manner, of plant migration-phenomena. 
Evidently, under the competition and struggle of the return, 
natural selection would operate as before in the development 
of new characters and the emergence of so-called new species. 
From the outline above it is apparent that a third and bio- 
logical factor must be added to the two already given, if one is 
to explain the differences between two regions supporting dis- 
tinct plant-populations. This factor, since it includes the ele- 
ment of time, might be called the time-factor, or better, simply 
“history.” 
