46 CAUSES WHICH FAVOUR OR [pt. i 



nature. If one carried seeds of Hydrocotyle asiatica (p. 30) from 

 Ceylon to the south of New Zealand, and planted them there, 

 they would probably refuse to grow, yet nature has gradually 

 acchmatised the species to both regions. Many species range 

 4000-8000 feet vertically in the Himalaya; seeds from the higher 

 levels produce plants much more at home in Europe than seeds 

 from the lower levels, and might spread much more rapidly in 

 cooler climates than the latter. Travel may be much slower in 

 a vertical direction, where conditions change comparatively 

 rapidly, than in a horizontal. 



Finally, we must go on to consider what are probably the 

 most important positive causes favouring or hindering species 

 in their dispersal; barriers are obviously negative. These causes 

 may also be classed in general as ecological, depending on 

 some peculiarity inherent in the plant itself, often described as 

 being an "adaptation" to something or other. We have already 

 considered in Chapter ii one of the most important of these 

 —the method of dispersal of the plant— and must now go on 

 to deal briefly with the others. In my published papers I have 

 perhaps not allowed enough for ecological barriers, but I am 

 not sure that they are sufficiently permanent to do more than 

 delay spread, rarely to completely stop it. 



Take, for example, the wide differences seen between trees, 

 shrubs, and herbs. The flora of the wetter tropical and southern 

 regions of the globe, and of large portions of the north, prior to 

 the great clearances made by man in recent times, consisted 

 mainly of trees. These had, it is true, more or less of herbaceous 

 undergrowth, but there was comparatively little open country 

 covered with herbs suited to a life exposing them to the sun and 

 the wind. Even in much of Europe, Asia, and North America, 

 that is now covered with herbaceous or shrubby vegetation, 

 there appears to have been forest over a great part of the country 

 during Tertiary times. 



It used to be generally supposed that the Angiosperms com- 

 menced as herbs and that trees were a later de\'elopment, but 

 this view is now usually reversed, and the herbaceous form is 

 looked upon as the younger. The change of view dates largely 

 from a paper by Sinnott and Bailey (99) in Avhich they marshal 

 the evidence from paleobotany, anatomy, phylogeny, and geo- 

 graphical distribution, etc., showing that it all points in the 

 same direction, to the conclusion that herbs on the whole are 

 the younger form of vegetation. 



