Geologie Correlation.— Ward. 45 
as it is expressed in modern scientific language, less completely 
differentiated than they are at the present day. The consequence 
is that we are all the time changing from one genus to another, 
and from one family to another as the evidence accumulates. 
Now there is no doubt that the later forms of life, both animal 
and vegetable, have developed from earlier forms, and_ these 
transition stages in the paleontologic record enforce this truth far 
more strongly than any facts in the living faunas and floras of 
the globe. Assuming then that the later floras are derived from 
the earlier ones, it is of the utmost importance, not only to the 
botanist, but also to the geologist, to: trace these ancestral rela- 
tionships of plants, and this can be done with considerable suc- 
cess. Therefore in the preparation of a table of distribution we 
must not confine our attention exclusively to the species which are 
found in the group to be compared. In fact so variable are these 
ancient forms that it would be impossible to do this. It would be 
very misleading to be guided exclusively by the names given to 
the species. The forms differ in different localities by such slight 
divergences that the personal equation of the describer would 
vitiate such a calculation. Some would join similar forms from 
different localities, others would separate them as distinct species, 
and the history of the nomenclature of paleobotany is merely a 
record of these apparently conflicting determinations, but which 
in reality after all, merely show that the forms are more or less 
closely related, although they can never agree in all respects. 
This, therefore, is the difficult part of the preparation of a 
table of distribution, viz., to select not only the species which 
are universally regarded as identical in two or more horizons, but 
also such as are believed to be related to those of the group under 
consideration. There is danger on the one hand of leaving out 
important related species, and on the other hand of introducing, 
as related species, those which really have no affinity. Without 
dwelling upon the details of this difficult part of the task, it must 
be assumed that the paleobotanist, if skilled in his craft, possesses 
that judgment which will enable him to distinguish the truly re- 
lated species from those which are only apparently so. To the species 
then which all admit to occur at more than one horizon or in more 
than one place, and which we will, for the sake of brevity, de- 
nominate identical species, must now be added to those which are 
related to species of the group, and which may be ealled related 
