CH. v] HINDER THE DISPERSAL OF SPECIES 49 



reason to believe that given sufficient time, and no interference 

 by man, forest would once more replace the open herbaceous 

 vegetation of the damper parts of the globe. 



Other types of habit may have entirely different effects upon 

 spread. Water plants can obviously only spread so long and so 

 far as there is water available (leaving out of account in this 

 place all negative factors like barriers of temperature, etc., 

 already considered above), parasites can only spread with their 

 hosts, saprophytes only with the presence of the necessary pro- 

 ducts of decay in which they live, epiphytes with the presence 

 of sufficient moisture, etc. Halophytes can spread wherever the 

 ground is sufficiently salt, mangroves where it is muddy and 

 covered by a quiet sea at high water. Climbers as a rule can 

 only go where there are plants sufficiently tall upon which to 

 climb. Xerophytes or plants of dry climates, once formed, will 

 be able to advance into dry country until the drought becomes 

 too great for them to survive, and so on. 



So long as a plant remains of average (mesophytic) type, 

 suited to an average damp climate and good water supply, it 

 may have an enormous territory possible of occupation if only 

 no barriers interfere, while a plant that becomes very specialised 

 in these respects may be limited in its capacity for spreading to 

 little more than the small area upon which it commenced. As 

 Thiselton Dyer says (94. p. 311), "The Nemesis of a high degree 

 of protected specialisation is the loss of adaptability," 



General evidence seems to indicate that it is not improbable 

 that in the Tertiary period the world as a whole was better 

 suited to mesophytic vegetation than at present, and hence it 

 is not unlikely that the earlier species not only gained in the 

 mechanical way described on p, 34, but also found fewer 

 barriers to their spread. Later formed species, on the other 

 hand, as they could not survive if not exactly suited to the con- 

 ditions in which they were evolved, would be increasingly likely 

 to find themselves with climatic or other ecological barriers to 

 further spread at no great distance away. A progressive speciali- 

 sation of climate and other factors seems to have been going on 

 in the world since the Tertiary period, the comparatively damp 

 and uniform climates of the latter being replaced by every 

 variety from very damp to very dry. Hence the more recent 

 species tend to become more and more specialised to match the 

 climates. To quote Guppy, "when one finds Salsola Kali upon 

 the Devonshire coast, upon a Chile beach, and upon the uplands 



W.A, 4 



