FEBRUARY 1, 1901. | 
history of the species, at once points to the 
superior importance of embryological data 
as the real link through which primitive 
forms may be united ; but as we approach 
these primitive forms it is to be observed 
that the differentiation of types becomes 
less and less clearly defined until they 
merge in the ancestral form, and this must 
hold true of extinct types as well as of those 
now existing. While on the one hand such 
approximations may greatly aid in the so- 
lution of given problems, on the other they 
serve to defeat this end by reason of the 
difficulty experienced in their recognition 
and in a demarcation of their precise limi- 
tations. 
In the progress of development many of 
the more primitive types of life have dis- 
appeared, while those of advanced organi- 
zation have survived, and thus many of the 
gaps revealed by the study of existing 
species are of a more or less permanent 
character, and can only be bridged hypo- 
thetically. It isin emergencies of this kind 
that we instinctively turn to geological rec- 
ords, and in paleobotany seek the only 
available means of solving the difficulty. 
But here again, the complete obliteration of 
perishable material in the process of petri- 
faction, the breaking-up of the original 
body into widely separated fragments which 
can be correlated, if at all, only after the 
most prolonged and arduous labor, and the 
difficulty of recovering plant remains, even 
though they may be present in the rocks, 
constitute obstacles to the progress of knowl- 
edge in this direction, of the most formi- 
dable character. Hence from a botanical 
point of view, our knowledge is extremely 
imperfect ; but when we take into consid- 
eration the serious nature of the difficulties 
involved, the substantial progress which 
has been made during the last seventy 
years, and more particularly during the 
last two decades, affords ground for much 
satisfaction. 
SCIENCE. 
In this connection we may note the some- 
what suggestive observations of White to 
the effect that ‘‘ The rapid development and 
series of changes or modifications, presented 
by the plants of the Pottsville Series of 
both Pennsylvania and West Virginia, point 
to the possibility that in this series we may 
eventually discover facts of the deepest sig- 
nificance relative to descent, but the present 
fragmentary nature of the material recoy- 
ered, which consists altogether of portious 
of foliage, will not warrant final conclusions 
in this direction.”’ 
The relative sequence of the Pterido- 
phytes and Spermatophytes, as also of the 
Gymnosperms and Angiosperms, the Mono- 
cotyledons and Dicotyledons as exhibited in 
geological succession, is substantially the 
same as that derived from the study of liy- 
ing forms. But with reference to their 
points of contact and the relations of the 
more subordinate divisions of these groups, 
especially with reference to the Thallophytes 
and Bryophytes, the evidence of geological 
records often proves altogether inadequate 
and conveys no very exact idea of the real 
phylogeny. As living plants constantly ap- 
proximate to the ancestral type as we de- 
scend in the scale, so likewise a similar ap- 
proximation must be exhibited in earlier 
geological time, and this slight differentia- 
tion of primitive forms, which are very im- 
perfectly known to us through mere frag- 
ments of the original plants, is one of the 
most prolific sources of error in paleobotany, 
and has resulted in the constant shifting of 
plants from one position to another. 
On theoretical grounds the Thallophytes 
must have flourished in the very earliest 
periods of, the earth’s history, and their re- 
mains should occur in rocks of the Lauren- 
tian age, but if we leave out of considera- 
tion those very problematical forms which 
have sometimes been held to represent 
plants from the early paleozoic, nothing 
recognizable appears until the Silurian, in 
