Chapter I — 5 — Scope and Method 



species inhabit areas separated from one another but lying within the 

 area of another related species, or they are separated by a region unoccu- 

 pied due to the dying out of intermediate species, or, in case they al- 

 ready differ sufficiently as regards conditions of habitat, overlapping 

 areas. Such species naturally differ greatly from one another mor- 

 phologically, and are usually not linked by any transitional forms. 



These first two categories of species appeared in northern and cen- 

 tral Europe in the post-glacial period in contrast to the third group of 

 species, which existed in the Tertiary Period and were preserved during 

 the Glacial Period either in southern Europe or beyond its boundaries, 

 whence in unaltered form they penetrated Europe proper only after 

 the recession of the glaciers. 



From the foregoing it is clear that phylogenetic taxonomy can de- 

 rive much from the geographical distribution of species in its task of 

 determining their relationships. Historical plant geography, on the 

 other hand, can determine, on the basis of the relationship between 

 species, the history of their origin and migrations, and thus approach 

 its chief task, the elucidation of the history of floras. 



Relation to Paleogeography: — Paleogeography is a new branch of geo- 

 graphical science. One of the numerous methods by the aid of which 

 paleogeography approaches a solution of its tasks is the paleo-bio- 

 geographical method, founded on a study of the former distribution of 

 living organisms. Paleontological data, due to their chance and in- 

 adequate nature, are not capable of solving problems as to the former 

 distribution of plants and animals on the earth's surface, which make it 

 necessary for the paleogeographer to seek in biological data for that 

 which geology fails to give him. Consequently, biogeography consti- 

 tutes one of the necessary bases for paleogeography. 



However, not all branches of the geography of living organisms are 

 equally useful in paleogeographical research. Of exceptional signifi- 

 cance for the geography of former periods of the earth's history is the 

 historical geography of plants and animals. The latter, to a consider- 

 able degree, have outlived the various changes which have taken place 

 on the earth's surface during their long existence, but these changes 

 cannot but be reflected in the present distribution of organisms. The 

 biologist in studying the latter stumbled on a number of facts, to ex- 

 plain which he took recourse, to some extent without having sufficient 

 geological data, to conclusions and h5^othetical propositions as to the 

 former configuration of and connections between the continents, which 

 in many cases subsequently proved to be confirmed by geological in- 

 vestigations and now are generally accepted as established facts. 



The study of the present areas of plants and animals, the elucida- 

 tion of the causes which brought them about, the drawing of con- 

 clusions as to the past history of these areas — all these factors in the 

 study of the distribution of organisms, constituting the essence of his- 

 torical biogeography, make up one of the cornerstones on which paleo- 

 geography is based. Paleogeographical data, on the other hand, 

 constitute a basis for conclusions as to the historical geography of 

 plants and animals. 



