108 CORRELATED VARIATION. [CHAP. V 



laws of correlation and variation, independently of utility and 

 therefore of natural selection, than that of the difference between 

 the outer and inner flowers in some Compositous and Umbelli- 

 ferous 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 Compositse countenances this 

 idea; but with the Umbellifene, 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 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 differences in shape, which are not always correlated with 

 any difference in the corolla, can be in any way beneficial : yet in 

 the Umbelliferse these differences are of such apparent impor- 

 tance the seeds being sometimes orthospermous in the exterior 

 flowers and coelospermous 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 systema- 

 tists 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 



