584 Plain, Prairie, and Forest. [ October, 
of forests over a considerable portion of the Mississippi Valley 
must be sought for. 
Let us next inquire whether there is anything in the distribu- 
tion of the rain-fall throughout the year or from season to season 
which may possibly have a peculiar influence on the vegetation. 
That this is the case, and that this is, in fact, the predominant 
cause determining the existence of the prairie, is the theory ad- 
vocated by Mr. J. W. Foster, at some length, in his work en- 
titled The Mississippi Valley. Mr. Foster thus states his views: 
“ Wherever the moisture is equable and abundant we have the 
densely-clothed forest, wherever it is unequally distributed we 
have the grassy‘plain, and where it is mostly withheld we have 
the inhospitable desert.” That the last of these three dicta is 
true may be at once admitted. No one will deny that some 
moisture is necessary to the growth of vegetation, whether it 
consist of trees, shrubs, or grasses. The other parts of this state- 
ment, as we contend, are entirely erroneous. And no better in- 
stance can be given of the fact that an equable and abundant 
distribution of moisture does not always clothe the country with 
dense forests than that of the vicinity of Chicago itself, where 
Mr. Foster’s book was written. Here we have one of the finest 
prairie regions in the world, absolutely destitute of trees, and 
yet in the full enjoyment of an abundant precipitation, and in 
the immediate vicinity of an immense sheet of water. For 
Chicago itself, indeed, the statistics of rain-fall are very defect- 
ive, but such as they are, they are entirely unfavorable to Mr. 
Foster’s hypothesis, Points in the immediate vicinity of that 
city, where observations have been taken for a series of years, 
show an annual average rain-fall of from thirty-six to fifty inches, 
pretty uniformly distributed through the year, as will be seen 
farther on. An excellent instance, on the other hand, of a dense 
growth of trees combined with the most unequally distributed 
rain-fall which is possible is furnished by the western slope ° 
the Sierra Nevada of California, whose magnificent forests arè 
well known, as also is the fact that there is no precipitation 
there at all for six months of the year, nearly the whole of the 
rain-fail being limited to three months. And, lest it may be 
thought that melting snow keeps the ground moist during the 
summer, it may be added that the heaviest forest belt of the 
Sierra is quite below the line above which snow rests for any 
considerable time, and that the soil in that belt is usually pe” 
fectly dry at the surface, and even dusty, for six months of the 
year, and often much more. 
