OBSERVABLE PROPERTIES OF EACH SPECIES 11 



is a consequence of plasticity and depends entirely on the 

 conditions of life. In both examples plasticity is prominent, 

 yet every living species is plastic to a certain degree. 



Other well-known examples of remarkable plasticity are 

 the so-called light and shade leaves of many plants (for in- 

 stance, Fagus sylvatica) ; the variation in the leaves of several 

 thistles according as they are developed in a dry or in a moist 

 atmosphere ; remarkable variations in the colours of a number 

 of butterflies ^ produced by external influences (temperature, 

 food) affecting the caterpillar or the chrysalis, etc. 



It seems as if many biologists did not pay much attention 

 to the above facts. They continue to give us descriptions of 

 species and anatomical contrivances as if plasticity did not 

 exist. Plasticity, however, is in itself a very important pheno- 

 menon, which deserves to be investigated quite independently 

 of the question whether the observed variations may become 

 hereditary or not. 



§ 13. — COMPLEXITY. — In certain species, which are also 

 called variable or polymorphic, the observed variation is inde- 

 pendent of external causes and depends on the fact that the 

 species under consideration consists of two or several distinct 

 hereditary forms or subspecies {elementary species) which are 

 brought together under one name. EXAMPLE : Solanum 

 nigrum is variable with regard to the colour of the ripe fruit. 

 In many localities (on the Continent) specimens with black 

 berries and others with green berries are found side by side 

 under the same conditions of existence. We know, on the 

 other hand, that each of the colours mentioned is transmitted 

 by inheritance (DE VRIES).^ We may conclude, therefore, 

 that S. nigrum is a COMPLEX species, including two sub- 

 species {melanocarpum and chlorocarpum) . Very probably 

 5. nigrum includes more than two sub-species. We see, indeed, 

 in BENTHAM'S Handbook of the British Flora (i866) that 



" in Britain, the stem ... is usually glabrous or nearly so, 

 but on the Continent often hairy or rough at the angles," 



that the 



" berries . . . are . . . usually black, but sometimes, 

 especially on the Continent, green, yellow or dingy red," 



and further, that the species is 



" varying so much in warmer regions as to have been 

 described under more than forty names " {loc. cit., p. 332). 



It is only by means of experiments that we can hope to 



1 Certain of the variations alluded to have been observed fortuitously in 

 the state of nature and described as very rare varieties. 



*HUGO DE VRIES, die Mutationstheorie (Leipzig, Veit und Co.), vol. ii. 

 (1903), pp. 156 and 170-171. 



