560 THE HISTORY OF BIOLOGY 



difficult exploration succeeded in elucidating the reproductive process of the 

 eel — a problem that many before him had tried to solve in vain. 



Touching the continental life -forms, the classification into large geo- 

 graphical regions already drawn up has on the whole been retained, and 

 both plant and animal geographists have for the most part devoted them- 

 selves to the study of conditions within smaller areas belonging to these re- 

 gions. Among investigators of this category in the sphere of animal life may 

 be mentioned the explorer Karl Semper (1831-93), professor at Wiirzburg, 

 who studied the problem of the life-conditions of animals from various points 

 of view. 



Plant geography: its floristk and fnorphological courses 

 In the field of plant geography, research has taken especially two courses, 

 a systematical, which is ultimately based on Linnxus's observations and 

 theories in connexion with the distribution of the plant species, and a mor- 

 phological, which has its origin in Humboldt's theories on the morpho- 

 logical association of different vegetable types with different countries and 

 forms of landscape. These two tendencies have exerted a mutual influence 

 and have, each in its own way, been influenced by the doctrine of descent 

 and its attempt to explain the origin of species out of conditions of geo- 

 graphical distribution. And at the same time valuable results were gained 

 by the comparison between the distribution of existent plant-forms and that 

 of the corresponding genera and species of earlier geological periods. All 

 representatives of modern plant-geography have been compelled more or less 

 to take these conditions into consideration. It is still possible, however, to 

 trace two main tendencies in this sphere, which nevertheless incessantly 

 touch and cross one another. The first of these, the systematic or floristic, 

 which rests upon the systematic entities, treats of the distribution of the 

 species within larger or smaller areas and their variations in different parts 

 of one area under the influence of certain factors. It endeavours to find out 

 the causes of the changes in the character of species in certain localities and 

 countries and for this purpose studies the migrations of species, such as oc- 

 cur through the distribution of land and sea in recent times and through 

 the changes that have taken place in the distribution in earlier ages, in so 

 far as it has been possible to trace these shiftings of the world's surface. 

 Further, it examines the distribution of extinct and fossilized species, from 

 which those of our own time may possibly have originated. Special interest 

 has been devoted to the immigration of plants in those parts of Europe that 

 were once visited by the glacial period, as well as those vegetable remains 

 in the mountain ranges of the polar regions that give evidence of a previous 

 warmer climate there. 



^ Morphological or oecological plant-geography does not investigate the 

 nature of the flora, but of the vegetation. It works, not with species, but 



