DERIVATION OF THE FLORA AND FAUNA 333 



of waifs and their descendants. What AxELROD says about migration probabiHties 

 during different geological epochs is of greater interest. 



Since plants are controlled largely by climate, and since climate has been changing 

 during geologic time, it follows that plants comprising different communities have had 

 different possibilities at different times . . . probabilities for long-distance migration were 

 much higher for tropical plants in the Eocene than they are to-day. Temperate forest 

 species had a m.uch higher probability from late Cretaceous to middle Tertiary, it is 

 low now. Steppe plants had a higher probability during Pliocene than now. Desert spe- 

 cies have a higher probability to-day than at any time before. 



His conclusions are drawn from the size and area of populations shifting with the 

 extension of climatic regions. 



For my own part I have expressed my opinion on overseas migration in the 

 Pacific on various occasions [2JI, 318, 248) and I am not going to repeat the dis- 

 cussion here. My general conclusion was that the effect of transoceanic migration 

 has been largely overestimated. 



GUPPY (121), who allowed birds, winds and currents to stock all oceanic 

 islands with plants, arrived at the conclusion that this traffic was a thing of the 

 past and that migration had practically ceased altogether. I expressed my doubts 

 that it had ever been effective, in any case with regard to seed-plants. 



I have already remarked that Setctiell laid stress upon what he called the 

 CEB (climatic-edaphic-biotic) factor complex. The main difficulty for the vagabond 

 plants was not to cover the distance, be it ever so great, but to become a successful 

 member of a community already established in the place where it happened to 

 alight, and this difficulty increased as time went by; most surfaces of the earth, 

 he says, are already stocked with closed vegetation, making it impossible for new 

 arrivals to gain a foothold {2ig. 300). His ideas are clearly expressed in 218 (\). 874). 



As the islands have become more and more completely stocked each with its quota 

 of plants and animals and have undergone various vicissitudes, particularly of elevation, 

 erosion, etc. its hospitality to migrating germules necessarily has become less and less, 

 the Biotic factor has become more complex and the Edaphic factor has also suffered change. 



In my view the result could just as well be the opposite, for these "vicis- 

 situdes", emergence, erosion, volcanic activity and so forth create new soil, a more 

 varied topography, a multitude of different habitats, all of which ought to give 

 newcomers increased opportunities to get established. 



With Setchell, Andrews underlines the importance of CEB; genera 

 expected to occur in Hawaii but absent "were not amenable to germination 

 and survival after transport". 



Long before Setciiell, J. D. Hooker, Wallace and others had paid 

 attention to the obstacles for the successful establishment of newcomers, WALLACE 

 believed that St. Helena had become stocked with plants during early Tertiary 

 time; later there was no room left, and the flora had changed so completely that 

 no plant was recognized as an insular form of a continental species. 



We meet with Setchell's line of thinking in a recent paper by W. B. Taylor 

 {26J. 572). In recent volcanic islands are many unstocked habitats to begin with, 



