334 PLANTS, SEEDS, AND CURRENTS 



important principle is probably concerned in the fact that the 

 more lowly organised Sphagna do not respond to the law to the same 

 extent as the Carices. It may be that this arises from the greater 

 capacity for dispersal of the spores of Sphagnum than the fruits of 

 Carex ; but all such advantages would, I think, be discounted in the 

 run of the ages. If we look at the table we notice that the northern 

 hemisphere is almost large enough for the complete differentiation 

 of Carex, but it is too small for Sphagnum ; and it would seem that 

 the lower plants need a larger area for evoking the full effects of 

 the differentiating process than the higher plants. It might even 

 happen that the world is not large enough for a particular group of 

 plants, and that a chaotic confusion of affinities in a geographical 

 sense would arise which would disappear in a world twice the size. 

 This involves the principle that lowly organised plants would be 

 less plastic than higher plants in the same area, or, in other words, 

 they would respond less to changes of conditions. The point that 

 our world may not be large enough for the lower plants should not 

 be forgotten, and some interesting deductions could be drawn 

 from it. 



It will be seen from these pages that free use has been made of 

 the materials supplied by Warnstorf and Kiikenthal in their mono- 

 graphs in the Pflanzenreich on the Sphagnacece and the Caricoidece. 

 Buried deeply in the mass of data contained in this splendid series 

 of publications there lies the romance of plant- distribution, which 

 the diligent student can unearth, should he possess the inclination 

 and the patience. If any success attends my efforts to present as 

 illustrating real living problems the facts so laboriously collated by 

 the German investigators, I shall have paid back a little of my debt 

 to the authors of these monographs. 



Explanatio v of the Table. — Though the table is mainly self- 

 explanatory, it should be remarked that the materials as arranged 

 in the monographs do not always lend themselves for precisely the 

 same treatment, which explains the varying treatment in the columns. 

 In the first place (A) all the species of North America and Eurasia 

 are dealt with, excluding those of the subtropical and tropical 

 zones. In the second and third places (B and C) only those of the 

 west hemisphere are utilised. 



The insular factor comes so much into operation in the northern 

 tropics, and there are so many disturbing influences affecting a 

 comparison between the tropical mainlands of the east and the west 

 in the northern hemisphere, that, except in the mainland of North 

 America, the warm latitudes have been disregarded. Islands, large 

 and small, introduce the effects of isolation in a way not presented 

 by a continent; and the disturbing influences of the insular factor 

 are again referred to in a later page of this chapter. The necessity 

 of excluding the island from this table is well exemplified in the case 

 of Sphagnum. If we were to include Japan in the Eurasian region 

 and the Malagasy province in Africa, we should find that half of the 

 endemic Eurasian species (thirty-three in all) did not extend outside 

 Japan, and that one-third of the African endemic species (forty- 

 seven in all) are restricted to Madagascar and the Mascarene Islands. 



