DISTRIBUTION 



325 



with huge land-masses radiating southward and wide oceans stretch- 

 ing northward, distribution becomes chiefly a problem of the north. 

 We cannot get over this fact by raising problems in the south. They 

 could only be subsidiary. The increasing differentiation of floras 

 with distance from the Arctic pole leave us no choice in the matter. 

 It cannot, therefore, be a subject for surprise that of the great land- 

 masses of the south each tells its own story, and that the discontinuity 

 between genera and species, so frequent in the south, diminishes as 

 we approach the Arctic area. 



Any theory of distribution will have to explain the abandonment 

 of share in the struggle on the part of whole groups of plants and 

 animals that found a sanctuary long ago in the southern lands of 

 South America, South Africa, and Australia. It will have to face 

 the fact that these three extremities of the great land-masses diverging 

 from their common centre in the north have become for plants and 

 animals " cul-de-sacs," as Thiselton-Dyer terms them, from which 

 there is no escape. It will have to explain why South America, 

 South Africa, and Australia have become the abodes of lost causes, 

 of causes that have been fought for and lost in the north. 



The Zoological Standpoint. — Zoologists have often appealed 

 to the hypothesis of a north polar centre of dispersal in explanation 

 of their difficulties. Dr. Scharff, in his recent work on The Distribu- 

 tion and Origin of Life in America (pp. 23, 427), refers in this connec- 

 tion to the views of Allen, Dahl, Haacke, Tristram, and Wilser, only 

 to reject them as untenable. 44 There have been " (he contends) 

 44 scores of great centres of dispersal in the world, and from them 

 streamed forth new forms in every available direction. Northern 

 animals advanced southward and southern forms northward, aided, 

 no doubt, by the ever-changing conditions of climate and the gradual 

 evolution of oceans and continents." 



Yet, if we allow for the occurrence of subsidiary centres, it can 

 be scarcely said that these views are radically inconsistent with 

 those advocated in these pages. Standing by themselves they can 

 hardly do other than make the investigation of distribution a study 

 of tangled results which may lead us anywhere. The method of 

 reconstructing the land-surface from the facts of distribution is the 

 least promising of all the modes of attacking the problem. The 

 opposite plan is here followed. The very thoroughness of Dr. 

 Scharff' s work appears to me to emphasise the hopelessness of ever 

 solving problems of distribution by taking up the ends of the tangled 

 skein. We must begin with a few general and simple premises that 

 are beyond dispute, and work forward. Working backward seems 

 to be the method least likely to evoke order out of the reigning chaos, 

 and for one among many reasons that we do not yet know the real 

 significance of many of the lesser facts of distribution, being often 

 utterly at variance with one another in our interpretation of them. 



The Geological Standpoint. — Let us for the moment forget 

 much about distribution, and start with the fact that there existed 

 in Tertiary times in the Arctic regions a subtropical climate and a 

 subtropical vegetation. Let us add to this the second fact that this 

 vegetation is not to be found within the Arctic Circle now, but in 



