Chap. I] THE FORMATIONS 189 



4. LIFE OF THE PLANT-COMMUNE IN THE 

 FORMATIONS. 



The various plants composing a formation indubitably stand in the 

 most manifold relations to one another as well as to the animals — worms, 

 insects, birds — that inhabit the formation. The question regarding the 

 nature and mode of action of these relations promises to yield most 

 important conclusions bearing on the oecological explanation of the 

 formations, but has hitherto been but rarely approached, and then only 

 in individual cases l . The floristic branch of geographical botany, on the 

 other hand, has indirectly yielded valuable material by the compilation 

 of lists of species that are constantly found growing together. Thus, 

 according to Flahault, there are always found accompanying Ouercus 

 Ilex in France, thirteen other species of plants, amongst them Cistus 

 monspeliensis, Lavandula latifolia, Thymus vulgaris ; whilst Fagus sylvatica 

 is always accompanied by the following amongst other species : Vaccinium 

 Myrtillus, Rubus Idaeus, Oxalis Acetosella, Mercurialis perennis. Hock 

 has drawn up such lists for several German formations. Naturally they do 

 not hold good for all regions nor for all kinds of soil, as to each agglomera- 

 tion of external factors there must correspond a definite grouping. 

 This fact obviously does not take from the value of such compilations, 

 at least if they are accompanied by accurate data regarding climate 

 and soil. 



To the same category of questions belongs an inquiry into the cause of 

 the social growth of some species and the invariably isolated appearance 

 of others. It appears, however, superfluous to inquire more closely into 

 the hypotheses that have been put forward in regard to these matters, 

 for they have as yet no solid foundations, except in the case of a few 

 tropical formations which will be discussed further on 2 . 



1 Schimper, op. cit. 



2 See upon this question de Candolle, op. cit. ; Warming, op. cit. p. 106 ; especially 

 Brandis, op. cit. 



