186 THE QUANTITATIVE METHOD IN BIOLOGY 



higher values (5 or 8, or both). Here the influence of the con- 

 ditions of existence is as indubitable as in the above-mentioned 

 experiments with Chrysanthemum carinatum. 



LUDWIG 1 has studied the variation of the number of 

 marginal florets in Chrysanthemum leucanthemum. Series of 

 specimens collected from certain localities showed the 13-floret 

 form to be the commonest, whereas the 34-floret form was 

 predominant in other localities. It would be premature to 

 conclude from these facts, without more information, that two 

 different subspecies (hereditary races) exist among the material 

 collected by LUDWIG (§ 130). 



§ 129. — INFLUENCE OF SELECTION UPON THE 

 VARIATION STEPS IN CHRYSANTHEMUM SEGETUM. 

 — DE VRIES sowed the mixed seed of Chrysanthemum segetum 

 obtained from twenty different botanic gardens. The terminal 

 flower-heads of each of the ninety-seven healthy plants obtained 

 were examined, and were found to contain the following numbers 

 of marginal florets 2 : — 



Florets . . 12 13 14 15 16 17 18 19 20 21 22 

 Specimens . i 14 13 4 6 9 7 10 12 20 i 



This two-humped curve (Fibonacci series of the second degree) 

 resembles certain two-humped curves of Chr. carinatum 

 obtained under unfavourable conditions or by examining 

 flower-heads of lateral branches. According to DE VRIES, 

 the observed discontinuity indicated the presence of two forms, 

 a 13-ray form and a 21-ray form. The seeds from 12- and 13- 

 ray specimens were collected and sown next year, the terminal 

 flower-heads obtained therefrom having the following numbers 

 of florets 3 : — 



Florets 

 Specimens 

 Florets 

 Specimens 



8 9 10 II 12 13 14 15 

 2 I o 7 13 94 25 7 

 16 17 18 19 20 21 22 

 7120300 



" That is to say, all trace of the 21-ray form had been elimin- 

 ated and a nearly pure 13-ray form obtained. That this was so 

 was proved by sowing the seed of some of the 12-rayed plants 

 obtained on this occasion in the following year. It was then 

 found that the frequencies of occurrence of flowers with various 



1 Quoted after H. M. VERNON, loc. at., p. 47. 



^ Archiv fiir Entwickelungsmechanik, Bd. II., p. 52, 1896. (Quoted after 

 VERNON, loc. cit., p. 50.) 



It is worth noting that in this curve the limits coincide approximately with 

 two variation steps, just as in Ceniaurea cyanus. (See § 127, p. 182.) 



^ Here the limits coincide once more almost exactly with two variation steps. 



