190 BROOKLYN BOTANIC GARDEN MEMOIRS 
continent, extending in an east and west direction, formed barriers 
which prevented their migration southward and there they became 
extinct. In North America, however, with its mountain systems 
extending in a north and south direction, migration to more congenial 
regions was possible and here they continued to exist. Their present 
isolated geographic distribution was, therefore, determined long ago, 
by a combination of climatic changes and topographic features, and is 
not a modern phenomenon that can be satisfactorily explained by 
present conditions alone. 
Incidentally it may also be pertinent to recall that the genus 
Sequoia enjoys the unique distinction of having been found in the 
fossil form previous to its discovery as an element in our existing 
flora. Cones and leaf-bearing twigs, representing what we now know 
as the genus Sequoia, were found in Europe and were described and 
figured (but not, of course, under the modern generic name) before 
the living trees on our western coast were discovered. This fact, 
however, can hardly be cited as an instance in which paleobotany has 
been of assistance to botany, inasmuch as it involves the question in 
nomenclature whether or not the generic name first applied to the 
fossil remains should have precedence over that subsequently given to 
the living trees. 
I shall not attempt, in this paper, to discuss the debt which botany 
owes to those paleobotanical students who have made special studies 
of the internal structure of fossil plants, and thus determined exact 
botanical relationships along lines of modern morphological investi- 
gations. This is a relatively recent phase in the development of 
paleobotany and the results attained are familiar to us all. The dis- 
covery of the extinct class or order of plants, the Pteridosperms or 
Cycadofilicales, and its taxonomic relations to the Pteridophytes and 
Gymnosperms, is due to their labors, as is also the determination of 
the exact affinities of many extinct families and genera with those now 
living. They have filled in the details of the broad phylogenetic 
sequence outlined by the earlier paleobotanists and they represent 
the field of work in which botany and paleobotany are most closely 
and intimately related today and in which it is impossible to dissociate 
them. 
