6 THE PALEONTOLOGY OF MINNESOTA. 
[Correlation of floras. 
In considering the question of the correlation of the plants recognized in the 
succession of the floras since their origin, one forcibly arrives at the same conclusion, 
the impossibility of explaining by antecedents the characters ot the vegetation of 
the Middle Cretaceous, or rather the presence of the dicotyledons as its essential 
element. From the beginning and in closely following the march of the vegetation, 
we find an evident degree of parentage between the groups which disappear and those 
which follow them. Thus the affiliation of the ferns of the Devonian to those of the 
Catskill group ; then to those of the Subcarboniferous, is easily followed up into the 
Permian, and still, by gradual modification through the ages, to the present epoch. 
The great Lycopodiacee,—Sigillaria, Lepidudendron, etc., gradually take a more marked 
place in the vegetation of the paleozoic times, have the highest degree of predomi- 
nance in the Carboniferous, have their time of decline in the Permian, but continue 
to be represented up to the present epoch by plants of the same kind but of small 
size. The conifers also, which distinctly appear in the Permian by peculiar forms, 
gradually becoming more predominant, constitute the essential vegetation of the 
Jurassic, still remain in the present flora under somewhat modified forms. The 
Cycadew, apparently as old as the Lycopodiacee, follow the same march of development 
traversing the Carboniferous as an essential constituent of the vegetation, declining 
in the Permian and by modification of some of their characters passing to the Cycadew 
which then follow the same march as the conifers. There is, indeed, between the 
Cordaitee and the Cycadew a marked difference, but the mode of gradual transform- 
ation between plants which have such great analogy of characters may be easily 
conceived. After following the gradual variations of types through the palzozoic 
time one may follow them still from the Cretaceous and see them also continued 
upon the dicotyledons from their appearance through the cenozoic ages, to the pres- 
ent epoch. Hence all the groups of vegetables appear from their origin as linked 
together by a kind of putative affihation ; but nothing like that can be seen to fore- 
tell the appearance of the dicotyledons in the Middle Cretaceous ; the parentage is 
derived from them in the hereafter, but none can be found in the past. 
As we see it in the Dakota group, in the formation of Atane in Greenland, and 
also in the Middle Cretaceous of Europe, the Cenomanian fora appears in its distri- 
bution and in the general character of its groups, as a complex assemblage of vege- 
table types developed under the acting forces of a long series of ages. Though 
a number of species are found identical at the different localities, the groups are 
generally different in characters. For example, the four species of Populus of 
Greenland are of the section of the coriaceous poplars, and of these, none is found in 
the Dakota group. Of the seven species of Quercus described from Greenland, two 
