180 CORBEL ATED VARIATION. [Chap. V. 



attributed to the pressure of the involucra on the 

 florets, or to their mutual pressure, and the shape of the 

 seeds in the ray-florets of some Composite countenances 

 this idea ; but with the Umbelliferee, it is by no means, 

 as Dr. Hooker informs me, the species with the densest 

 heads which most frequently differ in their inner and 

 outer flowers. It mi^ht have been thought that the 

 development of the ray-petals by drawing nourishment 

 from the reproductive organs causes their abortion ; but 

 this can hardly be the sole cause, for in some Compositae 

 the seeds of the outer and inner florets differ, without 

 any difference in the corolla. Possibly these several 

 differences may be connected with the different flow of 

 nutriment towards the central and external flowers : we 

 know, at least, that with irregular flowers, those nearest 

 to the axis are most subject to peloria, that is to 

 become abnormally symmetrical. I may add, as an 

 instance of this fact, and as a striking case of correlation, 

 that in many pelargoniums, the two upper petals in the 

 central flower of the truss often lose their patches of 

 darker colour ; and when this occurs, the adherent 

 nectary is quite aborted; the central flower thus 

 becoming peloric or regular. "When the colour is 

 absent from only one of the two upper petals, the 

 nectary is not quite aborted but is much shortened. 



"With respect to the development of the corolla, 

 Sprengel's idea that the ray-florets serve to attract 

 insects, whose agency is highly advantageous or neces- 

 sary for the fertilisation of these plants, is highly 

 probable ; and if so, natural selection may have come 

 into play. But with respect to the seeds, it seems 

 impossible that their differences in shape, which are not 

 always correlated with any difference in the corolla, 



