24 MR. J. BALL OTS^ THE T'LORA 



eflScient in the present condition of things. No doubt tornadoes 

 carry many seeds to considerable heights, and if they travelled 

 far with currents of equal violence, might carry them across 

 spaces of many hundred miles in width ; but, if at all, such a 

 possibility can scarcely exist in temperate latitudes. If winds 

 were efficient for the purpose, I feel sure that we should find 

 evidence of the fact in the much wider oceanic distribution of 

 species having small seeds or winged appendages. 



On the other hand, it appears to me that Mr. Wallace has not 

 sufficiently dwelt on the predominant influence of migratory 

 birds in the dispersal of plants, where physical obstacles prevent 

 their gradual diffusion. If it be true that many facts of distri- 

 bution may possibly be accounted for by ocean-currents carrying 

 floating timber or ice-rafts laden with soil, it remains true that 

 most of these facts arc at least equally well explained by the inter- 

 vention of birds ; whilst for numerous other facts transport by 

 birds affords the only probable explanation. I do not know 

 whether it has been remarked that, as between regions in the 

 temperate zone and others nearer to the pole, the chances of the 

 transfer of species by birds from higher to lower latitudes is very 

 much greater than in the opposite direction. As a rule, birds 

 winter in the milder climate and resort to higher latitudes in 

 spring or early summer*. In autumn, when seeds and fruits are 

 mature, they carry them to a milder region, where, under favour- 



able conditions, these may establish themselves. If we admit 

 the probability of an Antarctic centre of distribution for the types 

 of vegetation which we are agreed to call Antarctic^ but suppose 

 those types to have originally reached the Antarctic region from 

 South America or Australia, we find it very difficult to conceive 

 the means by which they could hare reached the polar continent; 

 while if that continent were their original home there is no diffi- 

 culty in understanding their subsequent diffusion in a northerly 

 direction. 



It may be reasonably objected that a similar argument might 

 "be urged against the primitive peopling of an Antarctic continent 

 by the ancestors whence the special Antarctic genera must have 

 been derived ; and the answer to this objection requires a very 



Professor G-iglioli has called my attention to the fact that some birds mi- 

 grate from low latitudes in summer to higher latitudes in winter. Such cases 

 are rare and exceptional, but tliey may serve to explain some anomalous facts 

 of plant dietribution. 



