204 INTRODUCTION TO PLANT GEOGRAPHY [CHAP. 



seasonal (exhibiting seasonal dimorphism, as in the case of closely 

 related forms differing in their times of development). 



Most ' systematic ' vicariads, consisting of pairs or sets of the 

 higher taxa which are vicarious, belong to the first or geographical 

 category, a good example being afforded by the various races of 

 Bracken {Pteridium aquilinum agg.) inhabiting different parts of the 

 world. There are also plentiful examples of physiographic vicariads 

 inhabiting lower and higher altitudes, respectively — for example the 

 Wood and Alpine Forget-me-nots {Myosotis sylvatica and M. 

 alpestris agg.), and the Common and Alpine Timothys (Phleum 

 pratense and P. alpinum s.l.). As examples of ecological vicariads 

 growing in different habitats and characterizing different communi- 

 ties, we may cite : in fresh and mainly salt marshes, respectively, 

 the Bulrush and Glaucous Bulrush [Scirpiis lacustris and S. tahernae- 

 montanii) ; in soils with high and low available water, respectively, 

 the Water and Wood Avens {Geum rivale and G. urbanum) ; and in 

 calcium-rich and calcium-poor soils, respectively, the Yellow Moun- 

 tain and Brook Saxifrages {Saxifraga aizoides agg. and S. rivularis 

 agg.). Such vicariads are commonly intraregional, not infrequently 

 growing as closely together as their habitat requirements permit, 

 whereas the geographical ones tend to be more widely regional. 



We have already indicated that vicariads, at least of the lower 

 taxonomic orders, tend to evolve chiefly about the periphery of a 

 migrating ' parent ' taxon. Here new characters are particularly 

 prone to arise by mutation or other chromosomal change, and help 

 the better-adapted offspring to survive under conditions which are 

 less favourable to the parent. Thus autopolyploidy, the pheno- 

 menon of multiplication of a plant's own chromosome set, may result 

 from extreme habitat conditions and be accompanied by evident 

 changes in the plants involved, autopolyploids often having very 

 definite geographical ranges differing from those of their ancestors 

 which possessed the normal (diploid) number of chromosomes. 

 When the changes accompanying polyploidy are very marked, a new 

 species may be constituted, which is often a vicarious one. Vicariads 

 may also arise as a result of hybridization, which is frequently accom- 

 panied by a multiplication of chromosomes (allopolyploidy) ; or they 

 may be a consequence of mere local differentiation resultant on 

 changes in climatic or other habitat conditions in some part or parts 

 of a plant's range. As a result, the initial species may ' break up ' 

 into a number of vicarious ones, or of subspecies with distinct 

 geographical ranges. 



