PART II 



THE OBSERVABLE PROPERTIES OF EACH SPECIES 

 ARE PRODUCED BY REACTIONS OF ITS LIVING 

 SUBSTANCE 



§ 10.— POSSIBLE AND OBSERVABLE PROPERTIES.— 

 The living mixture of each species is able to produce numerous 

 reactions which bring about the observable properties (so- 

 called characteristics). In each reaction internal {specific) 

 causes, which depend on the composition of the Uving mixture, 

 and external causes, which depend on the environment (con- 

 ditions of existence), play a part. The possible properties of 

 each species are very numerous ; each individual exhibits only 

 a part of them, according to the conditions of existence under 

 which it has been developed, the other properties being latent. 

 What we usually call the characteristics of a species are those 

 properties which become observable under ordinary circum- 

 stances of life. It is in this sense that the descriptions in the 

 floras and the faunas and also in anatomical and morpho- 

 logical hterature are to be understood. If the conditions are 

 modified, some of the described characteristics become latent 

 and other properties appear. 



Therefore, if we wish to discover all the possibilities (potenti- 

 alities, possible properties) of a species, we must collect and 

 examine specimens developed under all the different external 

 conditions under which the life of the species is possible. 



§ 11.— RANGE OF POSSIBILITIES. PLASTICITY.— 

 The range of possibilities is very different in one species from 

 what it is in another. I take as first example three species 

 which are characteristic of the alpine flora : Soldanella alpina. 

 Rhododendron ferrugineum and Leontopodium alpinum. When 

 these plants are cultivated in the plains (for instance, in the 

 Botanic Garden at Ghent), where the conditions are different 

 from those which prevail in high altitudes, they behave in 

 quite a different way. Soldanella is not at all or hardly modi- 

 fied, but it dies down after a certain time : the limits of external 

 conditions under which its existence is possible are rather 

 narrow.^ Rhododendron may be kept in the plains for an 



' This seems to be the case with many vegetable species (for instance, several 

 very rare terrestrial orchids) which are only found on certain spots where 

 peculiar conditions prevail. 



