Chap. I] THE FORMATIONS 189 



4. LIFE OF THE PLANT-COMMUNE IN THE 

 FORMATIONS. 



The various plants composing a formation indubitably stand in the 

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

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

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

 mportant conclusions bearing on the oecological explanation of the 

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

 n individual cases \ The floristic branch of geographical botany, on the 

 )ther hand, has indirectly yielded valuable material by the compilation 

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

 iccording to Flahault, there are always found accompanying Quercus 

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

 nonspeliensis, Lavandula latifolia, Thymus vulgaris ; whilst Fagus sylvatica 

 s always accompanied by the following amongst other species : Vaccinium 

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

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

 ot hold good for all regions nor for all kinds of soil, as to each agglomera- 

 ion of external factors there must correspond a definite grouping. 

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

 t least if they are accompanied by accurate data regarding climate 

 nd soil. 



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

 lie social growth of some species and the invariably isolated appearance 

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

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

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

 ropical formations which will be discussed further on^. 



' Schimper, op. cit. 



' See upon this question dc Candolle, op. cit.; Warming, op. cit. p. io6 ; especially 



randis, op. cit. 



