Chapter II —19— History 



opposite case they remain distinct and are converted into new geo- 

 graphical races or new species. Conditions in former geological periods 

 — such as changes in the relative distribution of seas and dry land, the 

 breaking off of islands from the mainland, and frequent volcanic erup- 

 tions — facilitated to a considerably greater degree the formation of new 

 species than do the conditions now existing. 



This law of species formation took shape in Wagner's mind as a 

 result of observing the existence in nature of very closely related 

 species, often separated by comparatively small barriers, such as rivers, 

 mountains, gulfs, etc., or growing on isolated mountain peaks. These 

 so-called "vicarious" species arose as a result of the chance transport 

 of the basic form across the indicated barriers, where, under the in- 

 fluence of new conditions and of isolation, it became modified and was 

 transformed into a new race. The law of migrations, which provides 

 a key to the understanding of the areas of distribution of vicarious 

 species and of breaks in such areas, is of great significance for historical 

 plant geography. 



An outstanding event in the field of study of botanical geography, 

 and particularly of its historical phase, was the publication in 1878- 

 1882 of Engler's "Versuch einer Entwicklungsgeschichte der Pflanzen- 

 welt, insbesondere der Florengebiete, seit der Tertiarperiode ". The 

 very fact of such a work being published indicates the great progress 

 that historical plant geography had made by that time. For, as we 

 have already pointed out, at the time of de Candolle the mere putting 

 of the question as to the causes and regularities of the observed distri- 

 bution of plants constituted an untouched theme, avoided by most 

 authors on account of the difificulties involved. Moreover, in Engler's 

 work we find the principles and data of historical plant geography 

 applied not only for an explanation of the distribution of separate units 

 of the plant kingdom but also for an explanation of the development 

 and interrelations of the floras of the entire globe — a problem, as we 

 have seen, first tackled by Hooker. 



Later investigations by Engler himself and by his followers and 

 students contributed much that was new, necessitating certain changes 

 in Engler's views on the development of floras. One of Engler's 

 services was the utilization of data on the geographical distribution of 

 genera and species of plants for his conclusions on the history of the 

 development of floras. Such data he obtained from monographs on 

 selected genera or species, having as their aim to check kinships by a 

 study of areas of distribution. Alphonse de Candolle in his "Geo- 

 graphie botanique raisonnee" (1855) was the first to note the need of 

 utilizing geographic distribution in making monographic studies of 

 genera and families. Five years later, in i860, Stur presumably was 

 the first to apply this principle in his monograph on the genus Astran- 

 tia. This same year Borshchov (Borszow) published his monograph 

 on the Aral-Caspian species of the genus Calligonum, in which he gives 

 an analysis of their geographical distribution. 



Somewhat later, in 1869, Kerner published a remarkable work on 

 the dependence of a plant on its geographical distribution in connection 

 with the effect of soil and climatic conditions, based on a monographic 

 study of species of the group Tubocytisus of the genus Cylisus. Dur- 



