36 CAUSES WHICH FAVOUR OR [pt. i 



dispersal, and will react to their surroundings in much the same 

 way. But as yet we have no means of comparing the rate of 

 spread of species that are separated in systematic relationship, 

 and which may differ in many ways. Some may have powder- 

 like seed, easily carried bj^ wind, others fleshy fruits dispersed 

 by birds; some may be herbs, with a generation every year or 

 two, and a corresponding chance of frequent dispersal, others 

 may be trees with as much as twenty to thirty years between 

 generations; and so on. 



If the dispersal of plants depended simply upon their 

 "mechanism" to that end, it is evident that (working with 

 groups of species, and long periods) it would be almost a purely 

 mechanical process, the area occupied enlarging steadily with 

 the increasing age of the species; and of course each species 

 would probably progress at a different rate, those with good 

 mechanisms, or in good environment, or flowering while still 

 young, travelling more rapidly. After a certain period of time 

 the areas occupied by a set of different plants, say a Dipterocarp 

 tree (p. 19), a Leguminous tree, a Cruciferous herb, and a Com- 

 posite herb, all starting simultaneously on area represented by 

 1, might at a guess be, say, 2, 5, 10, and 100. But in actual life 

 many other causes come in to facilitate or delay the spread of 

 species, and it seems probable that delay, rather than accelera- 

 tion, is the usual result. This is chiefly the case, for instance, with 

 the actual physical features of the world, which we shall 

 consider first. 



Ojjen seas, for example, and even comparatively narrow arms 

 of the sea, like the English Channel, may offer practically in- 

 superable barriers to migration, only to be occasionally passed 

 by a few species, unless M^ith the assistance of man. An im- 

 portant point to remember is that such seas, or arms of the sea, 

 may be comparatively recent, or of very ancient standing in. 

 geological history, so that their total effect upon distribution 

 may be relatively small, or of very great importance indeed. 

 Once formed, however shallow or deep, a sea will offer much 

 the same obstacle, and the degree to which it obstructs passage 

 of species will to some extent depend upon the direction of any 

 currents that may traverse it. Further, even when it has become 

 wide enough, in the process of formation,' to stop some species 

 completely, others, by virtue of good dispersal mechanisms, may 

 be able to cross. 



Mountains, again, are of great importance. Considered merely 



