of drier years, and which are, therefore, more, or less, favorable to forest growth?
The historical argument may be illustrated by the following:
The valley of Aragua, in Venezuela, is shut in on all sides, and the rivers which water it, having no outlet to the sea, unite and form Lake Tacarigua. This lake during the last thirty years of the past century showed a gradual drying up, for which no cause could be assigned. In the beginning of the present century the valley became the theater of deadly feuds during the war of independence, which lasted twenty-two years. During that time land remained uncultivated, and forests, which grow so rapidly in the tropics, soon covered a great part of the country. In 1822 Boussingault observed that the waters of the lake had risen, and that much land formerly cultivated was at that time under water. The drying up of the river Scamander in the Troad, and the contracting of the Euphrates in its channel, may be referred to as illustrations of the same effect of the cutting down of forests, and of diminished vegetation. (Buchan's "Introductory Text-book of Meteorology," 1871, p. 50.)
Clearly, we have nothing beyond the merest hearsay evidence in all this, and absolutely no facts upon which to base a scientific conclusion. Again, in regard to Greece:
In the course of centuries, the forests have in large measure been destroyed . . . and with the passing of the trees the rainfall has decreased, so that during the summer months, when hardly a shower comes to moisten the parched earth, the country is for the most part extremely arid. (Clarence H. Young, Bulletin American Geographical Society, Vol. 32, 1900, p. 151.)
Those with even an elementary knowledge of the climatic zones will recall that Greece, like northern California and northern Africa, lies in the subtropical belt, whose dry, or even wholly rainless summers, depend upon the great controls of temperature and pressure and winds and storm-tracks, far and away beyond the reach of any such insignificant local agencies as a few trees.
Or again:
The rainfall (of Teheran) was formerly very much less, say up to 10 or 11 years ago; it then did not, I think, exceed five inches per annum, but it is now about ten. The great increase is no doubt due to the many gardens which have sprung up within the last 10 years in and outside the city, and perhaps also to the formation, 10 years ago, of a lake 50 miles south of Teheran. The lake has a length of 22 miles, and is from 3 to 6 miles broad. (A. Hontum Schwindler, Symons's Monthly Meteorological Magazine, Vol. 28, 1893, p. 145.)
This is a good example of the weakness of the historical argument, even when apparently based upon actual observations.
We might cite further the rather hackneyed examples from Trinidad, where the cause of a general but rather slight decrease in the mean annual rainfall for ten-year periods between 1862 and 1891 (from between 66.50 and 67 inches at the beginning of the period to slightly over 65 inches at the end) has been "said to be the disappearance of the forests"; from Kimberley, where the cutting down of trees to supply timbers for the mines is supposed to have had "most injurious effects on the climate," increasing the number of dust-storms, among other effects; from Ismailia, where tree-growth since the opening of the
