been but recently usurped by the spruce; and he supposed that the birch
had first succumbed to the force of the winds, and the spruce promptly
taken its place, since, as a general rule, an evergreen growth succeeds a
deciduous, and _vice versa._ We have any number of well authenticated
facts similar to this stated by Professor Agassiz, but we cannot give
place to them, in this connection, without greatly exceeding our limits.

Dr. Franklin B. Hough, in his recent "Report upon American Forestry," to
which we have already referred, says: "It is not unusual to observe in the
swamps of the northern states, an alternation of growth taking place
without human agency. Extensive tracts of tamarack (_Larix Americana_) may
be seen in northern Wisconsin that are dying out, and being succeeded by
the balsam fir (_Abies balsamea_), which may be probably caused by the
partial drainage of the swamps, from the decay or removal of a fallen tree
that had obstructed the outlet." The writer of this work resided for a
period of ten years or more in Wisconsin, and during that time traversed
extensive portions of its territory, both before and after it became a
state. As early as 1844, the extensive tamarack swamps of that region were
manifestly dying out for the want of the proper nutritious elements in the
soil, and the balsam fir rapidly taking its place, especially where the
accumulations of soil, resulting from decayed vegetation, were favorable
for its appearance. The drainage of the swamps had not been thought of at
that time, nor had the swamps themselves been disposed of, to any
considerable extent, by the federal government. They were subsequently
granted to the state for educational purposes, and afterwards purchased up
in the interest of speculative parties.

But the decay of the tamarack had really commenced long before population
found its way, in any considerable numbers, into that section of the
country; and the balsam fir had begun its usurpation, in many of the