202 INTRODUCTION TO PLANT GEOGRAPHY 



that their mutual exckisiveness is due to competition. From their 

 very nature, ecotypes often tend to be vicarious, as do the extreme 

 ' ends ' of chnes. In any event, the process of genesis of geographical 

 races seems to be the main basis of the formation of ' vicarious 

 areas ', at least among the lower taxa, and this and any subsequent 

 segregation tends to take place towards the periphery of the range 

 of the ' parent ' species, where the latter is in general least happily 

 adapted to the environmental conditions. 



Examples of vicariads are to be found in almost any modern 

 taxonomic monograph in which the series of closely-related entities 

 inhabiting independent geographical areas are commonly recognized 

 as subspecies or even species. These tend to be mutually exclusive, 

 although sometimes their areas may show some overlapping ; and 

 it is often a matter of opinion (as well as, of course, the degree of 

 difference they exhibit) whether they should be termed subspecies 

 or raised to the rank of species. This is one of the most persistent 

 sources of controversy between the ' lumpers ' and the ' splitters ' 

 (of species). Numerous instances are afforded by major species that 

 are represented by different minor species or varieties (which most 

 often seem best considered as subspecies) on the two sides of the 

 North Atlantic — as, for example, the European Royal Fern, Osmimda 

 regalis, and its North American subsp. spectabilis {see Fig. 64). 

 Polyploids, dealt with towards the end of the preceding chapter, 

 are often vicariads {see also p. 204). 



Here it may be well to quote Jordan's ' Law of Geminate Species ' : 

 ' Given any species in any region, the nearest related species is not 

 likely to be found in the same region, nor in a remote region, but 

 in a neighbouring district separated from the first by a barrier of 

 some sort or at least by a belt of country the breadth of which 

 gives the effect of a barrier.' Such pairs of twin or ' geminate ' 

 species (or subspecies) actually constitute vicariads, differing in only 

 minor characteristics that are of later origin than their common 

 characters. 



True vicariads (which have arisen from a common stock) should 

 be distinguished from false ones which have not this close genetic 

 relationship. Frequentlv these last are members of different sec- 

 tions of a genus that have developed similar life-forms through 

 ' convergent evolution '. 'Frue ^•icariads (and consequently the 

 areas they demarcate) mav be classified according to the manner of 

 their separation from one another into (i) horizontal (geographical), 

 (2) altitudinal (physiographic), (3) habitat (ecological), and (4) 



