E. V. Wulff —74 — Historical Plant Geography 



breaks up into a number of vicarious species, the origin of which may 

 not be mutational, since during the process of natural selection inter- 

 mediate stages may die out. 



As the result of the breaking up of an initial type into a number 

 of vicarious species, a geographical series of species is formed. In 

 studying the area of any of these vicarious species for the purpose of 

 understanding its origin, it is, therefore, necessary to investigate not 

 only this area but also the areas of all the other species forming 

 the series. As an example of such a geographical series let us take 

 the series Frutescens in the genus Caragana, as given in Komarov's 

 monograph (1908). The area of this series of species embraces all of 

 China (except its tropical part), eastern and northern Tibet, Chinese 

 and Soviet Turkestan, the western foothills of the Altais, West Siberia, 

 and the southern and central part of European U.S.S.R. Within the 

 limits of this area there is a series of vicarious species distinguished 

 from one another both morphologically and ecologically: Caragana 

 chamlagu, which grows in China proper, is replaced, as it approaches 

 MongoHa, by the more xerophytic forms, C. rosea, C. Leveillei, and C. 

 opulens. In Chinese Turkestan these are replaced by others still more 

 xerophytic — the desert species, C. polourensis and C. turfanensis. 

 In the region between Issyk-Kul and Zaisan there is a secondary center 

 of species-formation, from which extend radially the areas of C. lacta 

 and C. Camilli Schneideri, which have not spread beyond the boun- 

 daries of this region, and those of C. frutex and C. grandiflora, which 

 have spread far to the west. 



Relic Areas: — The relic concept in botanical geography may be 

 formulated as follows: A relic species is a remnant of a more or less 

 ancient flora having a relic area occupied by it at the moment of its 

 entrance into the composition of a given present-day flora, from which 

 moment the age of the relic is measured. A relic area is usually iso- 

 lated and often contracting and discontinuous, constituting a remnant 

 of a once-extensive area. These characteristics, however, are not al- 

 ways obligatory. A relic species — being, as Darwin so aptly put it, a 

 "living fossil" — is the embodiment of the historical development of a 

 flora. 



The difficult -problem as to whether a certain species is a relic and 

 to what period of time it should be referred may be solved by a study 

 of the fossil remains of the given species and by a determination of the 

 age of the deposits in which they were found, and, if such are not avail- 

 able, by indirect botanico-geographical methods. The latter should in- 

 clude a study of the areas of this relic species and of closely related 

 species, not only within the Hmits of the given flora but throughout 

 their entire extent, and also a general geographical analysis of the ele- 

 ments of the flora under study, which will elucidate the historical 

 stages of development of this flora and the date of entrance into its 

 composition of these or those elements. 



In order, however, to elaborate a concept of relic species that will 

 be generally accepted, it is necessary to come to an agreement regard- 

 ing certain disputed points in their interpretation. These disputed 

 points are as follows: 



