170 
in different portions of the land surface, 
whence they spread with a rapidity difficult 
to conceive at the present day, over the 
greater portion of the northern continents. 
An extended study of the Pottsville 
series of West Virginia offers an important 
illustration of the rapid differentiation of, 
and change in, floras during this period. 
White shows that ‘ The period of change in 
conditions of environment attending transi- 
tion from Lower Carboniferous marine to 
true Coal Measures formation, was marked 
by an extraordinarily rapid development 
and modification of plant species. Within 
a relatively short period the meager flora 
of the Devonian and Pocono is multiplied 
to the inexhaustively fecund and highly 
diversified flora of the Carboniferous, a de- 
velopment scarcely possible except in this 
division of organic life (plants), which is 
the most sensitive to climatic change or en- 
vironment, excepting, perhaps, the higher 
vertebrates. In the lower part of the 
Pottsville series many species show a rela- 
tion to the floras of the Vespertine or Cal- 
ciferous Sandstone series; in the middle 
portion many of the forms are unique, 
while in thickly developed sections it is 
only near the top of the series that we see 
occasional Coal Measures forms creeping 
an),’? 
Recent investigations, by the Committee 
of the British Association charged with an 
investigation of the Pleistocene deposits in 
the vicinity of Toronto, have brought out 
in a very convincing manner the fact that 
three distinct climatic periods may be rec- 
ognized. During the first the vegetation 
and the climate were comparable with those 
now found in Labrador and the more north- 
ern portions of the Province of Quebec. 
This was followed by a period during which 
more southern types of vegetation were in- 
troduced, and the climate was probably 
comparable with that now common to the 
Middle United States. A third wave drove 
SCIENCE. 
(N.S. Vou. XIII. No. 318. 
these plants southward again, and once 
more northern types of vegetation were in- 
troduced, but not quite so boreal in their 
character as the first, and these conditions 
have remained unchanged in the present 
day. 
From the considerations which have thus 
been passed rapidly in review, it would ap- 
pear that the advantage which such studies 
offer to the botanist is not very large, and 
that while the value of paleobotany to the 
geologist may, upon these grounds, be con- 
ceded to be great, the solution of questions 
bearing upon climate and geological succes- 
sion can only be regarded as of a secondary 
importance from a botanist’s point of view. 
The great problems which confront the bot- 
anist of to-day relate to the filling in of 
those gaps in plant descent which become 
apparent from a study of existing species. 
He is, therefore, called upon to complete, as 
fully as possible, our knowledge of that se- 
quence in development which is at present 
justified, not only on theoretical grounds, 
but also on the basis of observed facts. To 
this end all other branches of the subject 
are subordinate, and since paleobotany has 
already contributed in so many important 
ways to enlarge our knowledge in this di- 
rection, it is my purpose to ascertain how 
far the progress of the subject in America, 
during the last decade, has proved of real 
assistance, the directions in which aid may 
be looked for, and the limitations within 
which useful results may be expected. 
Our conception of the four great primary 
divisions of the plant world and of their 
subordinate branches is based upon the 
theory of succession in development, but 
our knowledge in this respect is far from 
complete, and here and there gaps occur in 
places where, on theoretical grounds, there 
should be perfect continuity in development. 
This defect arises from several causes. 
The recognized law that the history of 
the individual more or less fully repeats the 
