Chapter VII 



TYPES AND AREAS OF NATURAL 

 DISTRIBUTIONS 



In the last chapter we considered what Hes behind the geographical 

 distributions of plants as we see them today. We must now concern 

 ourselves with those distributions that appear to be natural, leaving 

 until the next chapter the ' artificial ' ones which have obviously 

 been made or modified by introduction or other interference by Man. 



Each different kind of plant has its own particular distribution or 

 range, which is dependent, as we have seen, on its history, migrational 

 ability, and adaptability. Indeed, it is doubtful whether any two 

 of the hundreds of thousands of different kinds of plants known to 

 science have precisely the same distribution ; and in any case 

 distributions are changing all the time. It is consequently impractic- 

 able, and wellnigh impossible, to consider such matters in detail ; 

 yet when a broad view is taken many interesting facts stand out, and 

 generalization may be valuable. For whereas any hard and fast 

 system of classifying ranges would be artificial, because it would not 

 reflect the natural diversity observed, some useful categories can, 

 and for practical purposes should, be widely recognized. 



The term area (or range) in plant geography is applied to the 

 entire region of occurrence of a particular taxonomic entity (taxon, 

 plural taxa) or vegetational unit (econ, plural eca). Within this 

 range it is often necessary to consider the local distribution, some- 

 times called ' topography ', which at best is no more nearly continuous 

 than are suitable habitats for the entity or unit in question. For 

 whereas climatic limits usually constitute the chief boundaries of 

 plants, local topographic, edaphic, and biotic factors are all apt to 

 have their effect — as will be explained and illustrated further in 

 Chapter X. This, albeit secondary, effect is usually considerable, 

 often drastically limiting the areas of plants, within the bounds 

 prescribed by climate, to those offering otherwise favourable 

 conditions. 



Mention should be made here of the hypothesis of ' Age and 



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