FEBRUARY 1, 1901.] 
leaves, even when they are presented in a 
complete condition—a difficulty which is 
enormously increased when the fossil hap- 
pens to be fragmentary, as is generally the 
ease. The errors arising from a disregard 
of such facts, have led to no end of con- 
fusion, and have been among the most 
prolific causes of extensive revisions. It is 
therefore clear that for botanical purposes 
such fragments have a very limited value, 
but as Ward very correctly observes, for 
geological purposes it is not so much a 
question of correct botanical determination 
as the correct recognition of a plant once 
named and associated with a given deposit. 
With these limitations before us, we can 
form a more correct estimate of some of the 
recent conclusions bearing upon the clima- 
tology of former geological times. 
The great analogy of climatic conditions 
recognized as existing between those which 
influenced the flora of the Dakota Group 
and those which now govern the vegetation 
of the North American continent, led Les- 
quereux to the conclusion that the flora of 
North America is not at the present epoch, 
and has not been in past geological times, 
composed of foreign elements brought to 
this continent by migration, but that it is 
indigenous, Its types are native, and the 
diversity of their representatives has been 
produced by physical influences. All the 
plants of the American Cenomanian, except 
those of Ficus and the Cycads, might find 
a congenial climate in the United States 
between latitudes 30° and 40°. 
An examination of the Tertiary flora of 
the Yellowstone has shown that the present 
flora of the Park has comparatively little 
relation to it and cannot be regarded as 
a descendant of it. Knowlton concludes 
that the former had its origin to the 
south, while the latter had its origin to 
the north, and that the climate prevailing 
in Tertiary times was, therefore, quite dif- 
ferent from that now known, and in all 
SCIENCE. 
169 
probability not unlike that of Virginia of 
to-day. 
A study of the Lower Carboniferous 
Measures of Missouri has led Mr. David 
White to draw attention to the uniformity 
of climate which prevailed over Hurope, 
within the Arctic Circle, in North America’ 
Asia and, to some extent at least, in the 
southern hemisphere during Carboniferous 
times, and the probability that the ex- 
tremely close relationship between the 
floras of the Culm, Millstone Grit and basal 
portions of the Lower Coal Measures in 
Europe and America point to wonderful 
facilities for plant distribution during Culm 
and early Mesocarboniferous time, facilities 
which, with the aid of an even climate, 
made possible the comparatively regular 
distribution and sequential order of prob- 
ably nineteen-twentieths of the genera, 
and an unknown proportion of the identi- 
cal species ; and he is disposed to consider 
that the conditions favorable to plant dis- 
tribution, and consequent comparatively 
homogeneous dispersion of the successive 
floras of the Northern Hemisphere during 
the period extending from the later Culm 
to near the middle of the Mesocarboniferous, 
have never been equalled since. That there 
was plant migration cannot for a moment 
be doubted. Yet the evidence of distribu- 
tion, of vertical range, of characteristic as- 
sociations, and of the succession of the 
floras, indicate such geographical uniform- 
ity of climate and such facility of intermi- 
gration over a minimum distance, as to 
justify us in regarding the astonishingly 
similar associations of identical or closely 
related genera and species which charac- 
terize each stage, zone or group of the Culm 
and Mesocarboniferous, as essentially con- 
temporaneous in all the basins of the 
Northern Hemisphere. From this evidence 
he draws the final inference that many of 
the species or genera of the Mesocarbonif- 
erous were, under local conditions, evolved 
