14 THE QUANTITATIVE METHOD IN BIOLOGY 



ordinarily rooting at their base, the ascending part being about 

 5 cm. high, with numerous leaves. In one ditch (between 

 Beernem and Ruyselede, in West Flanders) I found a form 

 with a creeping stem, distant, comparatively narrow leaves, the 

 extremity of the stem not being erect. The difference between 

 the two forms was striking and much more apparent than 

 between the numerous specimens of the aquatic form which 

 I have observed in many locahties in Flanders. Therefore I 

 surmise that the two terrestrial forms belong to two subspecies 

 which can hardly be distinguished in their ordinary aquatic 

 form. 



REMARK : When we want to ascertain whether two plants 

 belong to the same species (or subspecies) or not, we often 

 cultivate specimens of both side by side under ordinary con- 

 ditions. If a distinct difference is observed, we conclude that 

 there exists a specific difference between them. If the speci- 

 mens show the same properties, we are tempted to beheve that 

 they belong to the same species. The latter conclusion, how- 

 ever, may at times be erroneous. It is possible that certain 

 reactions of the living mixture and the corresponding visible 

 differences are only observed under unusual conditions. A 

 better method would be to have comparative cultures under 

 conditions as various as possible (soil of diverse composition ; 

 dry or wet ; shady or sunny position, etc.). In this way we 

 may hope that latent differential characteristics will become 

 visible, which have not yet been observed. The principle of 

 this method is, mutatis mutandis, adopted for the investigation 

 of lower fungi, bacteria, etc. It might render good services 

 to the study of critical species of phanerogams, mosses, etc. 



§ 18.— MONOTYPIC AND COMPLEX SPECIES. 

 SPECIES AND SUBSPECIES.— To sum up, the so-called 

 variabiUty (variation, polymorphism) of a species may depend 

 on two quite distinct causes (compare § 8a) : 



(i) The living mixture of a species is capable of reacting in 

 various ways according to differences in the external condi- 

 tions. I call this plasticity . 



(2) A species may be (and many species are) a complex con- 

 sisting of two or several subspecies differing from one another 

 by the qualitative composition of their hving mixture ^ (each 

 subspecies being, of course, plastic). I call this complexity. 



The distinction between plasticity and complexity is well 

 known. It may, however, be useful to recall it, because we 

 often apply the terms variability, variation and polymorphism 

 to both. This confusion occurs again and again in zoological 



* The chemical difference between two subspecies is smaller than between 

 two species. (See § 8a.) 



