Shaler.] 
266 
[March 4 
eluded, in the case of most parts of this field, for the reason that 
the prevailing course of the stream, is from the north to the south, 
which is entirely the case in the region west of Michigan where 
the transportation of these species to the north has been most ef- 
fectively accomplished. It is only in a small part of the field 
about the great lakes where this method of carriage would have 
had any influence upon the dissemination over the drift cov- 
ered surface. 
Tornadoes are doubtless agents which serve occasionally to con- 
vey the seed of our larger trees from south northward, the run 
of their paths is to the eastward with a slight inclination to the 
north. I am therefore not disposed to think that they could have 
accomplished any considerable part of the northward progress of 
these large seeded species of plants. Moreover, the march towards 
the pole of these forms is nearly as great in the eastern section 
of the United States, where these storms are rare, as in the coun- 
try where they most prevail. It appears to me therefore, that it 
is mainly to the natural spread of the seed, from the extremities 
of the boughs and by the carriage effected by rodents that we 
must look for the northward progress of these forms. Allowing 
that it requires an average of thirty years to bring a tree of these 
species to the point where it bears fruit, and that the average 
spread of the seed in each generation is about two hundred feet, 
we may estimate that it would require about twenty-five genera- 
tions for the form to extend for the distance of a mile. On the 
supposition that each generation advanced for the distance of two 
hundred feet, and required thirty years for the step, it would de- 
mand seven hundred and fifty years to traverse a mile; 15,000 
years for one hundred miles, or to traverse a distance of 400 miles 
a period of 300,000 years. 
It is evident that this period, determined by the rate of pro- 
gress of large seeds, is altogether excessive. It is clear that other 
agents of transportatiomhave been in operation. It appears to 
me that the abbreviation in time can probably be accounted for 
by the occasional more extended carriage of the seeds by rodents, 
by the varied conveyance of them by tornadoes, and perhaps the 
rare transplantation by the action of the primitive races of men 
who inhabited this country. Making allowance for the action of 
these occasional means of dissemination, the impression remains 
