ApEIL 17, 1903.J 



SCIENCE. 



617 



neigiiboring regions. The greatest floral con- 

 trast exists between the Hercjmian flora and 

 that of northwestern Germany; closer rela- 

 tions are manifest toward the east, south and 

 west. 



The species composing the German flora 

 are referred to eleven natural areas of dis- 

 tribution, among which are the boreal, Al- 

 pine, Ural, Pontic, Atlantic and Arctic. A 

 detailed study of the present distribution of 

 species belonging to these various areas ren- 

 ders possible a discussion of the paths along 

 which the existing floral elements of Ger- 

 many have migrated at different periods. 

 Naturally, the degree to which the elements 

 from a particular area become dominant de- 

 termines more or less the tone of the land- 

 scape in any given district. 



The body of the work is devoted to an ac- 

 count of plant societies, of which thirty-two 

 are distinguished, and to the distribution of 

 these societies and their character species in 

 fourteen natural districts of central Germany 

 The descriptions and enumerations are sc 

 definite and inclusive as to present for every 

 one of these districts a satisfactory picture 

 of floristic relations. The author's contribu- 

 tions and methods in this direction are so 

 well known as to render their present discus- 

 sion unnecessary. 



The fifth and last division of the book is 

 devoted to a consideration of the causes, past 

 and present, that have contributed to the 

 establishment and characteristics of the Her- 

 cynian flora as it is to-day. As a matter of 

 fact, ' Hercynia ' does not suggest a simple 

 unity as a vegetation region; the unity is 

 rather geographical, and there are included 

 within it a number of vegetation regions 

 which may lie alongside of each other in the 

 plain, and above one another on the moun- 

 tains. Immigrations have been controlled in 

 the flrst place by orographic structure and 

 edaphic conditions, determined by the sub- 

 stratum, which consists of crystalline rocks, 

 basalt, and, especially in the west, of Triassic 

 limestone. Climatic factors, in themselves 

 alone, and in connection with physiographic 

 features, and the chances of immigration 

 along natural favorable routes are also all tc 



be taken into account. It is particularly 

 difficult to form an exact estimate of purely 

 climatic influences on the delimitation of 

 Hercynian districts and landscapes. West- 

 ward from the Harz, for example, Atlantic 

 species have settled, favored by the greater 

 amount of moisture, while in the same lati- 

 tude eastward there is a great development 

 of Pontic groups on the dry triassic soils. 

 The same Ilex that grows wild in the neigh- 

 borhood of the Weser is more likely to freeze 

 in severe winters to the southeast of the Harz. 

 Certain cereals sensitive to excess of precipi- 

 tation, such as the finer varieties of barley, 

 yield the best harvests along the lower Saale, 

 but all these and numerous other well-known 

 facts are the result of a complex of causes 

 in which general climatic relations must be 

 recognized but are by no means the exclusive 

 factor. 



This leads naturally to a discussion of geo- 

 logical relations, which, though brief, is highly 

 suggestive, and is of the more value in that 

 the author, with such abundant data at his 

 disposal, attempts only in the most conserva- 

 tive way theoretical constructions that have 

 often proved of seductive interest. He holds 

 that there is no ground for the assumption 

 that Germany was ever in the condition of 

 Greenland of the present day. Eelicts, such 

 as Hymenophyllum and various other genera, 

 prove that the last time of glaciation in the 

 Hercynian hill country did not destroy all the 

 remnants of the preceding period. A histor- 

 ical succession may be recognized in which 

 Arctic tundra are followed by a northern 

 steppe flora, which in its turn gives place to 

 forest. Eelicts of these different periods are 

 still living together, and in some places have 

 formed remarkably mixed societies. 



Drude discusses in some detail the traces 

 of the ice age in subalpine heaths and moors 

 of the Hercynian Mountains, showing that, 

 with the advance of the ice, alpine species 

 as well as the old stock of Scandinavian forms 

 were driven southwards, that finally along the 

 border of the inland ice stretching from the 

 Elbe northeast through Prussia there must 

 have been an exchange of such species, so 

 that hill country, such as that of Hercynia, 



