n6 Correlated Variation. Chap.v; 



the outer and inner flowers in some Compositous and Umbelliferous 

 plants. Every one is familiar with the difference between the ray 

 and central florets of, for instance, the daisy, and this difference is 

 often accompanied with the partial or complete abortion of the 

 reproductive organs. But in some of these plants, the seeds also 

 differ in shape and sculpture. These differences have sometimes 

 been 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 

 Umbelliferse, 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 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 Composite 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 necessary 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 differ- 

 ences in shape, which are not always correlated with any difference 

 in the corolla, can be in any way beneficial : yet in the Umbelli- 

 fera3 these differences are of such apparent importance — the seeds 

 being sometimes orthospermous in the exterior flowers and ccelo- 

 spermous in the central flowers, — that the elder De Candolle 

 founded his main divisions in the order on such characters. Hence 

 modifications of structure, viewed by systematists as of high value, 

 may be wholly due to the laws of variation and correlation, without 

 being, as far as we can judge, of the slightest service to the species. 



We may often falsely attribute to correlated variation structures 

 which are common to whole groups of species, and which in truth 



