58 THE QUANTITATIVE METHOD IN BIOLOGY 



disturbed. The ordinary correlation is, of course, a consequence 

 of the fact that the conditions of existence are ordinarily favour- 

 able or unfavourable through the whole life (a few months) of 

 each specimen. 



1 have observed in Chrysanthemum carinatum remarkable modifications, 

 the explanation of which is found in the sensitive period. Dr C. DE BRUYKER 

 (Ghent) has observed similar facts. 



This calls for reflection, especially with regard to the methods 

 of studying correlation. Little attention has hitherto been 

 paid to this interesting question. 



THIRD EXAMPLE: In the ordinary blue subspecies of 

 Centaurea cyanus (observed in Flanders) I consider two prim- 

 ordia of the marginal florets : length and colour. In a young 

 bud the colour is white : this primordium is original and 

 transitory. The second period of the development (within the 

 bud) begins with the appearance of a very pale rose-purplish 

 colour, which is very probably produced by a transformation 

 of a previously existing colourless chromogen. The intensity 

 of the new colour increases up to a certain limit which I call 

 fale rose-purplish. This primordium is metamorphic and 

 transitory. The third period is initiated (still within the bud) 

 by a transformation of the pale rose-purplish into blue. The 

 intensity of the blue colour increases till the flower-head is 

 expanded. The primordium blue is metamorphic, arrested 

 and caducous (strictly speaking, it is deciduous, since it vanishes 

 by degrees during the post-floration) . While the three terms of 

 the property colour follow each other, the primordium length 

 increases continuously till the maximum is reached (this 

 happens a short time after the expansion of the bud). The 

 primordium length is original, arrested and persistent.^ 



THIRD EXAMPLE [continued). PLASTICITY.— I have 

 cultivated the blue Centaurea cyanus under very poor conditions 

 of existence (about seventy specimens in a seed-pan, the diameter 

 of which was about 30 cm. and which contained only sand). 

 The plants were dwarf ; most of them produced only one 

 terminal flower-head. The corolla of the marginal florets was 

 distinctly shorter, the blue colour hardly paler than under 

 ordinary conditions. We observe here a new example of the 

 independence of the primordia : a modification of the condi- 

 tions of life has here an important influence on the value of 

 the primordium length and an almost negligible influence on 

 the primordium colour. "^ (In Primula sinensis the reverse was 



^ In the development of the colours mentioned an interesting example of 

 gradation is observed. (See § 133. ) The succession of the colours is an example 

 of segregation. (Compare Viola, § 38, p. 46, and Myosotis, § 46, p. 54.) 



2 In the experiment mentioned, under unfavourable conditions, the adult 

 marginal florets were distinctly infantile with regard to the length, hardly 

 infantile with regard to the colour. 



