118 THE ORIGIN OF SPECIES 



etc.), are likewise on the whole the most abnormal in their teeth, 

 but there are so many exceptions to this rule, as Mr. Mivart has 

 remarked, that it has little value. 



I know of no case better adapted to show the importance of the 

 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 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 compositae countenances this idea; but with the 

 umbelliferae 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 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 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 nec- 

 tary 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 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 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 differences are of such apparent im- 

 portance^ — the seeds being sometimes orthospermous in the ex- 

 terior flowers and coelospermous in the central flowers — that the 



