194 THE QUANTITATIVE METHOD IN BIOLOGY 



It happens very often that distinctly different primordia 

 follow one another in the course of the individual development 

 according to a certain order a, b, c . . . the primordium c being 

 produced by a metamorphosis of b, and & by a metamorphosis 

 of a. The primordia a,b,c... may then be looked upon as 

 being successive terms of a series, which I call an embryological 

 series. Certain series of variation steps very probably represent 

 successive states of development ; but between a series of steps 

 by which the successive states of one primordium are repre- 

 sented, and an embryological series, which indicates the 

 developmental relations between several different primordia, a 

 fundamental difference exists. 



In Myosotis palustris the colours white, rose, blue (§ 46, p. 54) 

 are the terms of an embryological series which is observed in 

 many BoraginacecB. In the blue Centaur ea cyanus a similar 

 series exists, consisting of the terms white, rose-purplish and 

 blue (§47). 



In Myosotis versicolor the terms are white, yellow, blue. This 

 series recalls the embryological series of colours observed in the 

 subspecies of Viola tricolor mentioned in § 38. 



The transformation of a given term of an embryological 

 series into the next one may take place in different ways. In 

 Myosotis palustris the metamorphosis of rose into blue proceeds 

 in such a way that irregular blue spots appear on the rose- 

 coloured petals. These spots (the limits of which are rather 

 vague) coalesce more and more till the petal is blue. In 

 Centaurea cyanus, on the contrary, the metamorphosis of rose- 

 purplish into blue begins at the extremity of the petals and 

 proceeds gradually and regularly towards their base [gradation ; 

 see Part VIIL). 



The available information about the relations alluded to 

 in the present paragraph is hitherto fragmentary. Further 

 investigation of this subject is desirable. A more complete 

 acquaintance with the developmental relations between differ- 

 ent primordia and between the different values (variation steps) 

 of a given primordium might throw some light upon the rela- 

 tions between the terms of a given pair of properties in the 

 hybrids,^ and upon other biological problems. 



§ 134.— THE DIFFERENCES BETWEEN THE TERMS 

 OF AN EMBRYOLOGICAL SERIES MAY BE EXPRESSED 

 QUANTITATIVELY.— In Myosotis palustris the primordia 

 white, rose and blue being chemical properties, they may be 

 represented by chemical formulae, which are the expression of 

 quantitative notions. 



1 strict segregation or transitory values (goneoclinie) ; dominant and 

 recessive, etc. 



