4 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 69 



eastern Africa to the mountains of Abyssinia and thence to the 

 Central African ranges ; but that there may have been a pluvial 

 epoch (I suppose in interglacial or postglacial times) when the zones 

 suitable to these plants were much more nearly approximated than 

 now ; i. e., the alpine-arctic European plants would have descended 

 to much lower levels and reached a much more southerly limit in 

 Europe than now, while at the same time the conditions in North 

 Africa would be such that similar and therefore favorable climatic 

 conditions would occur at much lower altitudes than at present, and 

 that the seeds of the plants in question (most of them being small- 

 seeded plants) were transported either by strong northerly winds 

 or by the agency of birds, or both. 



I cannot quite think this theory fully adequate to explain the data 

 of this question of distribution, especially in view of the additional 

 facts evinced by the mosses dealt with in the present case. If the 

 occurrence of these alpine-arctic seed-plants is due to what I have 

 termed fortuitous transport, *". c, there was no general migration in 

 a southerly direction, but owing to the fact that similar conditions 

 prevailed simultaneously in southerly Europe and the highlands of 

 north-eastern Africa, at no very great distance away, aided by a 

 pluvial epoch, extending over the regions concerned, seeds of these 

 plants happening to be transported found a congenial resting place, 

 thence retreating farther south to higher altitudes as warmer and 

 more xerophytic conditions ensued — if this is the full explanation, 

 it appears to me that we might ask why did not the corresponding 

 interchange from south to north occur at the same time? Why 

 do we not find in the present European alpine-arctic flora isolated 

 instances of Central African genera, transported at the same time 

 from there to here? One does not see why there should not have 

 been southerly wind-currents adequate for transport equally with 

 northerly ones ; indeed, anything like a continuous or prevailing 

 northerly wind would seem to presuppose a counter-current, possibly 

 at somewhat higher levels, from a southerly direction. And if the 

 means of transport was, or was aided by, migratory birds, they should 

 in their return journeys have been equal carriers of a certain number 

 of representatives of the African flora to the European lands. 



On general grounds, therefore, I should have thought that Engler's 

 solution of the problem in some measure failed to satisfy the con- 

 ditions, inasmuch as it would seem to give all the requirements neces- 

 sary for a counter-exchange of southerly plants to Europe, which 

 does not appear to have taken place. And I venture to think that 



