TYPES AND AREAS OK NATURAL DISTRIBUTIONS 183 



Area ', which claims that the area occupied by a species is propor- 

 tionate to its age {i.e. the time it has existed). In spite of what 

 has just been said, this is often true and is indeed somewhat axiomatic, 

 as reconsideration of dispersal and migrational ' mechanics ' would 

 lead us to expect. For if two or more species with identical capacities 

 in these respects begin their migrations at different times, the earliest 

 starter will be found, at any particular time, to have extended the 

 farthest. Yet actually there are so many superimposed factors 

 causing complications, and such numerous and often striking excep- 

 tions, that the hypothesis is of very doubtful value, and at best may 

 be considered a mere generalized working one — cf. p. 209. 



Before proceeding to the main topics to be considered in this 

 chapter, we should explain the additional taxonomic concepts of 

 ecads, ecotypes, and clines. An ecad as understood in this work 

 is a plant type or form produced within the life-time of the individual 

 in response to a particular habitat factor. An ecotype is a distinct 

 race resulting from the impress or selective action of a particular 

 environment. A dine is a geographical or ecological gradient in 

 phenotypic characters [i.e. physical make-up). These entities are 

 below or outside the usual specific bounds but should be borne in 

 mind as exhibiting much the same geographical characteristics as 

 do the usually higher taxa which we are more prone to consider, 



' Continuous ' Intercontinental Ranges 



Except perhaps if it is very limited, the area of a taxon or of a 

 vegetational feature is never really continuous ; in reality all manner 

 of interruptions occur, resulting in some characteristic topography 

 (using this term as implying local distribution-pattern). Neverthe- 

 less we tend to refer to those distributions which involve spreading 

 over a whole territory as ' continuous ', at least provided the various 

 stations are not more widely separated than the normal dispersal- 

 capacity of the plants concerned. 



Among the most frequent causes of interruption is the lack of 

 suitable habitats, which indeed may themselves be widely separated 

 or sparsely distributed. In such circumstances it is a matter of 

 proportion, and consequently of opinion, as to whether a particular 

 range should be considered continuous or otherwise. Thus whereas 

 the Sea-beach Sandwort {Arenaria peploides agg.) is found on almost 

 all sea-shores of temperate and boreal regions, where its distribution 

 may in the wide sense be considered virtually continuous, it is 



