E. V. WulfE —52— Historical Plant Geography 



consider that such an appearance of a new geographical race or a new 

 species is not something foreordained in the very nature of the develop- 

 ment of a species, as might seem from Pachosky. It may take place, 

 as we have pointed out above, either as a result of great climatic 

 changes (and only then does a new race or species actually arise 

 in situ) or, and this is more common under present-day conditions, as a 

 result of the initial species during its dispersal coming upon habitat 

 conditions, climatic or edaphic, to which it is unaccustomed and which 

 induce its modification. Hence, the migration theory of the origin of 

 an area preserves all its significance, although it should be noted that 

 the term migration is often misused, an exaggerated and often in- 

 demonstrable significance being ascribed to it. 



We consider, moreover, on the grounds above outlined, that an area 

 may originate not only simultaneously over a large territory, as 

 Pachosky assumes, but also within the limits of definite centers. We 

 wish also to make clear that, even in speaking about that territory 

 within whose limits the process of species-formation embraces all indi- 

 viduals of the initial species, we have in mind not the entire area but 

 only that part of it which came under the influence of unusual (for the 

 initial species) ecological conditions, unless, of course, the entire area 

 came under such influence. 



Likewise we may explain the tendency of whole taxonomic groups 

 to break up into various races or species differing in color (Taliev, 

 1 91 5) as a result of the geographical isolation of different forms of a 

 polychromic species. Isolation plays a very important role in the 

 origin of new forms or new species. This is particularly evident in the 

 case of the uplifting of new mountain chains, the separation of islands 

 from the mainland, etc. Komarov (1930) has thus formulated it: 

 "The breaking up of the earth's surface into isolated patches of dry 

 land increases the number of species." The same idea has been ex- 

 pressed in a recent paper by Skottsberg (1938), who points out that, 

 when a mainland breaks up into an archipelago or group of islands, 

 the population of any given species on such islands breaks up into a 

 number of vicarious species (see Chapter V). Certain local micro- 

 conditions — soil, relief, etc. — may also isolate plants from one another 

 and serve to induce the appearance of new forms. 



But if we may accept, with the reservations outlined above, 

 Pachosky's area theory, this is entirely impossible as regards the so- 

 called theory of hologenesis (Rosa, 1931). According to this theory, 

 the evolution of a species and the direction of such evolution does not 

 depend on the influence of environmental conditions but only on in- 

 ternal causes. At a certain moment, suddenly and throughout the 

 entire territory occupied by a species, even if it be distributed over 

 the entire globe, the maternal species breaks up simultaneously in all 

 localities (regardless of different habitat conditions) into the same two 

 daughter species; each of the latter, in its turn, throughout the 

 entire territory occupied by them, breaks up into two species, etc. 

 Thus, the evolution of organisms proceeds by dichotomous division. 



Assuming uniform habitat conditions in ancient geological times, 

 Rosa considers that the very first, most primitive species had an area 

 embracing almost the entire globe. For a very prolonged period the 



