180 COKRELATBD VABIATION. [Chap. V. 



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

 in the ray-florets of some Composits countenances this 

 idea; hut with the UmbelliferEe, it is by no means, as 

 Dr. Hooker informs me, the species with the densest 

 heads which most frequently diSer in their inner and 

 outer flowers. It might 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 Compositee 

 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 be- 

 come abnormally symmetrical. I may add, as an in- 

 stance 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 be- 

 coming peloric or regular. When the colour is absent 

 from only one of the two upper petals, the nectary is 

 not qidte aborted but is much shortened. 



With respect to the development of the corolla, 

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

 sects, whose agency is highly advantageous or necessary 

 for the fertiUsation 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 cor- 

 related with any difference in the corolla, can be in 

 any way beneficial: yet in the Umbelliferae these dif- 



