700 KANSAS CITY REVIEW OF SCIENCE. 



ing with regard to their extent and frequency even among those who have lived 

 many years on the plains. My own inquiries, made of such persons, confirm the 

 above statement. 



2. All successful attempts to make trees grow on the plains have been made 

 in irrigated districts, which is of course no test of their ability to grow in districts 

 not irrigated. At least one unsuccessful attempt has been made, viz., that of the 

 Kansas Pacific Railroad Company to plant them along their line. 



3. It is true that three cottonwoods, Populus monilifera, P. balsamifera, and 

 P. balsamifera, var. angusiifolta, grow spontaneously along the valleys of streams 

 in some places, one of which, P. monilifera, is often found far out on the plains. 

 These, however, rather substantiate than disprove the theory here advanced, since, if 

 the timber were destroyed by fires, the cottonwoods would share the common lot 

 of all. In fact they furnish still stronger proof against the fire theory, for where 

 they are found is just where the most grass grows, and hence where the fires 

 would be most frequent and destructive. It is therefore safe to say that the gen- 

 eral absence of arborescent vegetation is not due, as on the more eastern prairies, 

 to annual fires. 



To say that the distance from actual forests is too great for trees to migrate 

 over the plains, would be not only to misapprehend the question, since they would 

 have been there from the first, but would be to betray ignorance of the facility 

 with which vegetation is known to traverse vast areas and find its way across 

 continents, and even hemispheres of the globe. No point on the plains is too 

 remote for the cottonwood to fail to reach it, provided the conditions for its 

 growth exist, and if it be said that the peculiar anemophilous seed of this genus 

 adapt it above other trees for finding a proper habitat, it may be replied that few 

 other trees live in such small areas, so widely separated from one another as to 

 require similar devices for scattering their seed. If the entire plain was capable 

 of supporting a general forest growth, such as once existed in the Eastern States, 

 the individuals of any species would be in such close proximity to each other 

 that propagation by the ordinary methods would be as easy there as here. 

 The whole subject may therefore be briefly summed up as follows : 

 Since the elevation of the Rocky Mountain range at the close of the Tertiary 

 age, the atmosphere, in the general easterly movement which it possesses at all 

 latitudes within the United States, has at all times lost the greater part of its 

 moisture by condensation upon the cold summits of these and the more western 

 ranges, so that by the time it reaches the great plains it is too dry for precipitation 

 except under unusual conditions. As it moves still farther eastward across a level 

 country, having river valleys and lake basins, it comes in contact with currents 

 from the north, the south and the east, brought there by the constant disturbances 

 of barometric pressure with which all are acquainted, and in this manner it grad- 

 ually becomes at length again sufficiently laden with moisture to yield portions of 

 it to the soil when condensed by currents of unlike temperature. This character- 

 istic becomes more and more marked with the eastern movement until the Missis- 

 sippi Valley is reached, in which and at all points eastward the rain-fall, varying 



