HOOKER LECTURE, 1922. 239 
The wide distribution in the Northern Hemisphere of Mesozoic Ferns, 
linked by different degrees of affinity to the families we have considered, in 
contrast to the prevalence to-day of these families in the Southern Hemisphere 
suggests comparison with the transtropical wanderings of northern genera 
when the gradual lowering of temperature which heralded the Pleistocene 
Glacial period was the compelling factor. In the Northern Hemisphere at 
least there is no evidence of a general Glacial period in the Mesozoic era. 
If in pre-Tertiary days there was a similar exodus from North Temperate or 
even Arctic regions, some driving force other than the pressure of glacial 
conditions must have been involved. The point I would make is this : do 
the facts support the assumption that during the Mesozoic era the tendency 
of plants, or more especially Ferns, was to seek new homes in the South ? 
The hypothesis of a southern migration has often been sugeested for earlier 
as for later periods, and Wieland goes so far as to assert that a boreal centre 
of creation—using the word in a strictly scientific sense—is ** an established 
certainty." Let us take two illustrations : the Cretaceous flora of Western 
Greenland was particularly rich in members of the Gleicheniacez : the 
Rhietic flora of Soathern Sweden was characterised by a wealth of develop- 
ment of ferns allied to the Dipteridinæ. 
At first sight it may appear inconsistent with geologi al data to assume a 
northern origin for the Gleicheniaceæ because pre-Uretaceous species of 
Gleichenites are recorded from localities far south of the Arctic Circle where 
they occur in strata older than those in Greenland. In Western Greenland 
there is no evidence of any submergence of the Archean mainland, not even 
of the fringe of it, in early Mesozoic times. Plants no doubt flourished on 
the land, but no sediments were accumulated that earth movements have 
rendered accessible to us. 
The absence of Gileichenites has not necessarily a special significance ; the 
genus may have been represented but not preserved. While a northern 
origin for the family is not entirely ruled out, it cannot be said that the facts 
warrant the conclusion that the Gleicheniaceæ began their career on an 
Arctic continent; they may have spread from some centre in Southern 
Europe. | 
Reference has already been made to the rival claims of Scania and Tonkin 
to be regarded as one of the earlier centres of development of the Dipteris 
type of Fern. On the whole I am disposed to think that it was from 
Scandinavia and other northern regions that Tonkin derived its flora. 
Whatever view we may hold on the degree of direct relationship between 
the Paleozoic and older Mesozoic floras, it is difficult to believe that the 
Gondwana continent was the ancestral home of the great majority of the Ferns 
which colonised with conspicuous success the Northern Hemisphere. The 
Permo-Carboniferous Glacial period had its effect upon the vegetation, and 
it is unlikely that the conditions were such as to favour the production of 
more than a very few new forms which afterwards played a leading part in 
