DERIVATION OF THE FLORA AND FAUNA 329 



of former facilities for exchange of biota are in themselves neutral in deciding which 

 alternative should be preferred without paying attention to the ambiguity of much of 

 the geological evidence and the grave geophysical difficulties (p. 8i). 



The ])rogress of knowledge of geophysics certainly seems to ])oint to thinking in 

 terms of land bridges and their coastal margins. 



The later findings of seismology, indicating three distinct rock-shells each prob- 

 ably capable of both vitreous and crystalline states, carry the possibility, as Jeftreys 

 has clearly insisted, of vertical movements on a major scale. Such strong vertical move- 

 ments are indeed not merely to be e.xpected; they are widely and unargubly evidenced 

 by the facts of geomorphology no less than those of stratigra])hical geology (p. 82). 



R. F. Joyce, The relalion of the Scotia Arc to Pangaea, pp. 82-88. 



Joyce made an attempt to reconstruct Wegener's Pangaea in Lower Palaeo- 

 zoic time. To him continental drift is a possibility only, he says that if Pa7igaea 

 did ill Jact exist, his arrangement of the actual continents and islands — as usual, 

 the Pacihc island world is not involved — at the opening of the Palaeozoic era is 

 more in accord with the known data than in Wegener's reconstruction (p. 87). 



Two biogeographers took part in the symposium. 



H. E. HlXTOx, The ]Vegener-I)u Toit theory of contiiiental disphicenietit and the 



distribution of ani}nals, pp. 74-79. 



HiNTOX rejects the liberal construction of bridges to suit the demands of 

 specialists. Beside the usual apprehensions he adds the following. 



A further objection to the past existence of transoceanic bridges ... is the nearly 

 complete absence of deep-sea deposits on the existing continents, since we would expect 

 some of the latter to have been also elevated from the sea floor (p. 75). 



This seems reasonable; nobody earnesth' proposes to fill the surface of the 

 globe with land, leaving no or little space for the water. Bridges of any consider- 

 able size cannot have been contemporaneous, and transgression on part of what 

 is now land seems inevitable, if tracts of ocean floor were exposed. We have 

 ample proofs that considerable transgressions occurred, but these ancient seas may 

 have been too shallow to be of much importance as a support of the bridge 

 hypothesis. HlXTOX thinks that we require relatively few inter-continental con- 

 nections, the most important being Brazil-Africa and Australia-Antarctica, but not 

 in the form of bridges — -of such, he admits isthmian and shelf-bridges, nothing 

 more. Thus, for those who insist on direct inter-continental contact, the only 

 hypothesis at their service is the drift theory, and he asserts that most of the 

 modern biogeographers accept the broad outlines of this theory, unless it is claimed 

 that sliding occurred long before the end of the Mesozoic. As we have seen, 

 however, this is exactly what has been claimed. For his own part he puts much 

 faith in chance dispersal across broad stretches of open water. 



R. Good. The distribution of the Flowering Plants in relation to theories of co7i- 

 tinental drift. 



After some remarks on the general distribution of plant families, grasses and 

 Compositae taking the lead almost everywhere — which seems quite natural in view 



