7] TYPES AND AREAS OF NATURAL DISTRIBUTIONS 209 



species during its dispersal often running into climatic and/or 

 edaphic habitat conditions to which it is unaccustomed and which 

 in time may lead to modification of the incipient area. From some 

 such beginnings further migration usually leads to the current area. 

 However, sometimes drastic change may lead to evolution in situ. 

 Especially in the cases of young species and of those that have had 

 their ranges reduced by relatively recent catastrophes, the area at 

 present occupied is only a part, and often only a very small part, 

 of what it might be. For, as we shall see in the next chapter, each 

 species tends to have, besides its actual area, a potential area which 

 may be demonstrated by artificial introduction and is often of great 

 practical importance in the regional allocation of crops. It can also 

 be of significance with regard to the nuisance caused by weeds. 

 Indeed, it seems probable that the majority of present-day ranges 

 are by no means complete so far as the occupation of areas of suitable 

 climate and soil conditions are concerned. Sometimes this incom- 

 pleteness is due, as implied above, either to an insufficient lapse of 

 time since the entity evolved, or to the basic inefficiency of its dis- 

 persal — or a combination of youth and inefficiency. But probably 

 it is more often due to historical changes such as glaciation, or to 

 the operation of boundaries set by physical barriers such as seas or 

 mountains or deserts, by ecological conditions, or by competition 

 with other species. 



Although there is a natural tendency for the areas occupied by 

 many plants to increase with age, which can be an important factor 

 in biogeographical considerations, the relationship of area to age is 

 by no means as direct as has sometimes been supposed. To be 

 sure, with some genera and species, especially in certain tropical 

 regions, the area of spread is roughly proportional to the age (as 

 was suggested by the now unpopular hypothesis of ' Age and Area ' 

 ■ — see pp. 182-3), but in others this is so far from being the case that 

 the area at present occupied gives little or no indication of age. This 

 is true, for example, where ancient fossils indicate a much wider 

 distribution than now obtains, as in the case of the giant Redwoods 

 {cf. Fig. 43). For actually, at least outside of some favoured regions, 

 there have been so many, often drastic disturbances that the general 

 situation appears to be that the size of an area occupied by a species 

 depends less on the age of the species than on other factors. These 

 include its adaptability and competition-rigour, the circumstances of 

 its genesis, and whether or not ecological conditions and any dispersal 

 mechanism or mechanisms have favoured successful migration. 



