226 R I K A R D S T E R N E R 



During the twenties, tiiirties and forties of the last century a number of very 

 important works appeared: Schouw's and Meyen's well-known s}nopses of the 

 objects and methods of phytogeography; the problems of the causes of the 

 distribution of species became a more universally treated as subject, as for 

 instance in works by Heer (1835), Unger (1836) and Thurmann (1849); in some 

 works historical phytogeography began even to become visible (A. P. De Can- 

 dolle, Heer). 



The climax of taxonomic phytogeographical research, in the sense of Humboldt 

 and Wahlenberg, was marked by the important activity of A. De Candollc and 

 Grisebach at the middle of the nineteenth centur\'. The latter may be said to 

 have deiuied the views of the time most sharply. 



Grisebach seeks the explanation of the distribution of species in now existing 

 forces, known from experience (for instance, 1882, p. 200). He dismisses all 

 » geological » causes for the present distribution of species, such as build on the 

 »hypothesis» that the present flora has arisen out of an earlier one. The species 

 have liad the power of spreading their seeds over the area where they are to be 

 found at the present time; and that area is limited by geophysical factors among 

 which tlie climate is the -most important one. Species may become extinct — 

 they may, for instance, be supplanted by other species — , and the extinction 

 ma\" take place with different rapidity in different parts of the area of distribution. 

 In this way the disjunctive distribution of species is explained. — (jrisebach was, 

 as is well known, one of the foremost opponents of the evolution theory of Darwin. 



Grisebach made detailed examinations of the distribution of species within minor 

 areas. One of them is expounded in the work »Ueber die Vegetationslinien des 

 Nordwestlichen Deutschlands» (1847). It is in this book that Grisebach has per- 

 haps exhibited most clearly his opinion as to the nature of the distribution limits 

 of species. A couple of quotations from it ought therefore to be given here: 



f>Sofern jene Grenzlinien den Vegetationscharakter der Gegcnd ausdrijcken, vvelche 

 sie umschliessen, nenne ich sic Vegetation.slinien. Das Areal einer Pflanze hort 

 also auf an ihrer Vegetationslinie. Fallen solche Linien in ihrer Lage mit clima- 

 tischen Linien, /. H. mit Isothermen, mit Linien gleicher Temperaturextreme u. 

 s. w. zusammen: so ist damit das Beweis gefiihrt, dass in den hierdurch ausge- 

 driickten climatischen Werthen die Ursache der ortlichen Begrenzungjener Gewachse 

 liegt» (p. 465), and 



»Die.se regelma.'^sige Gcstalt der Pflanzenareale weist darauf hin, dass die Ursache 

 der Vegetationslinien nicht in der Mannigfaltigkeit terrestrischer Bedingungen, 

 sondern in den weit regelmassiger in bestimmten Richtungen wachsendcn und 

 abnehmenden, atmospharischen Abstufungen liegt, welche die allgemeinen Erwar- 

 mungsgesetze der elastischen Iliille des ludkcirpers hervorbringen und wovon die 

 Meteorologie durch ihre mittleren, climatischen Werthe Rechenschaft giebt.» 



