6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



should just those species have been chosen out which, being sterile 

 plants, present special difficulties to such modes of transport, when 

 so many fertile species must have been present, the spores of which 

 would have been transmitted infinitely more readily ? 



And it may be again pointed out that most of the mosses involved 

 are markedly species of the colder, more boreal conditions of the 

 palaearctic region. Now if we have to postulate a very discontinuous 

 area at the migratory period, so that the transported plants had to 

 pass over numerous and considerable gaps of lower, warmer, and 

 drier land before reaching a suitable " pied-a-terre," it seems reason- 

 able to suppose that the species that would survive would be rather 

 those of the lower and more southerly type, those in fact more 

 capable of enduring subxerophytic conditions, whereas it would be 

 difficult to select — short of actual aquatic species — any more pro- 

 nouncedly hygrophytic mosses than most of those in question, while 

 Tetraplodon bryoides and Hylocomium proliferum var. alpinum 

 though less distinctly hygrophytic are exclusively alpine, and the 

 former at least would be quite unable to resist anything like xero- 

 phytic conditions. True, it might be argued that species of a less 

 pronounced hygrophytic nature may have been transported under 

 such conditions and may have been since crowded out, by the return- 

 ing African flora, from the lower altitudes of the African mountains 

 as the present climatic conditions supervened ; but to maintain this 

 contention would (since it implies the transport of a large number of 

 plants by fortuitous means) be still more to strengthen my position 

 that we should all the more expect under these circumstances to see 

 the remains in Europe of a counter-exchange of species from south 

 to north. 



I cannot help concluding, therefore, that a more continuous land 

 area under colder and more hygrophytic conditions than Engler 

 admits is postulated by the known facts, and that the practically 

 total absence of any counter-exchange from Africa to Europe pre- 

 supposes something much more definite in the way of a southerly 

 migration than has hitherto been recognized, difficult as it may be to 

 trace the land connection that would have provided the necessary 

 bridge of transit. 



It is possible that the working out of the flowering plants of the 

 expedition may throw further light on this problem of geographical 

 distribution ; but it is one in which to an unusual extent the lower 

 plants such as the Bryineae may be expected to prove the best wit- 

 nesses, and it is much to be hoped that further exploration will be 



