130 CORRELATED VARIATION. 



panied with the partial or complete abortion of the repro- 

 ductive organs. But in some of these plants the seeds also 

 differ in shape and sculpture. These differences have some- 

 times 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 compositse 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 compositse the seeds of the outer and inner flo- 

 rets differ, without any difference in the corolla. Possibly 

 these several differences may be connected with the different 

 flow of nutriment toward 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 color ; and when 

 this occurs, the adherent nectary is quite aborted, the central 

 flower thus becoming peloric or regular. When the color is 

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

 Dot 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 fertilization 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, can 

 be in any way beneficial ; yet in the umbelliferae these dif- 

 ferences are of such apparent importance — the seeds being 

 sometimes orthospermous in the exterior flowers and coelo- 

 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, 



