Chapter V —63— Types of Areas 



(1926) refers those species that have been encountered only once and 

 only in one place and that are, for the most part, mutants, being likely 

 to disappear as suddenly as they arose. 



Besides species whose endemism is the result of the history of their 

 origin and distribution there are species whose endemism is connected 

 with definite habitat conditions. The latter may be called ecological 

 endemics. As is well known, there are a large number of endemic 

 species which arose as a result of edaphic conditions, e.g., species con- 

 nected with cretaceous soils, such as Linaria cretacea, Silene crelacea, 

 Scrophularia cretacea, and Hedysarmn crelaceum; with sandy soils — 

 psammo-endemics, in the terminology of Lavrenko (1936) — such as 

 Agropyrum tanaiticunt (in the Don Valley) and species of the series 

 Centaurea margaritacea s.l. (in the Don and Dnieper Valleys); with 

 granite, serpentine, and solonchak soils. Moisture conditions are like- 

 wise a cause of the occurrence of endemic species; among such species 

 we may mention those that grow along the banks of rivers (Fursaev, 

 1937)- 



The determination of the type of any given cases of endemism, 

 particularly the division of endemic species into paleo- and neo- 

 endemics, is a very important factor in the analysis of a flora. In re- 

 tracing the history of a flora paleo-endemics may serve as important 

 guideposts. The paleo-endemic nature of a species is usually deter- 

 mined by its relic character, its geographic and often taxonomic 

 isolation, the absence of variation in characters, its narrow restriction 

 to definite ecological conditions, upon all of which depend the limited 

 size of its area and, to a great extent, its retrogressive nature. But all 

 these factors are not sufficiently objective to determine positively the 

 age of an endemic. For this purpose the use of cytological data has 

 been proposed, e.g., for the genus Festuca (Lewitsky and Kuzmina, 

 1927) and, more recently, for the germs Agrostis (Sokolovskaya, 1937). 

 Taking into consideration the circumstance that, when a genus com- 

 prises a polyploid series of species, those species having a small chromo- 

 some number are older than and constitute the initial forms for the 

 species having the larger chromosome numbers, we may use this charac- 

 ter in determining the relative age of any given case of endemism. 

 Endemic species with a small chromosome number may usually be 

 referred to paleo-endemics, while neo-endemics have larger chromo- 

 some numbers. For example, the endemics Agrostis planifolia (a Cau- 

 casian species) and A. hissarica (a Central Asiatic species) have 42 as 

 their diploid chromosome number, while the endemics A. Biebersteinii 

 (a Caucasian species) and A. Trinii (an East Siberian species) have 

 only 14. It is clear that the former should be regarded as neo- and the 

 latter as paleo-endemics. 



Vicarious Areas: By vicarious areas we mean areas for the most 

 part mutually exclusive and belonging to closely related species, differ- 

 ing only in a few specific characters and linked by the fact that they 

 derived their origin from one initial form. The formation of vicarious 

 areas has at its basis the process of the genesis of geographical races. 

 Speaking of vicarism, we have in mind chiefly species and geographical 

 races (subspecies), although there may also be vicarious sections of 



