132 ON THE ORIGIN OF SPECIES 



the corolla. Possibly, these several differences may be 

 connected with some difference in the flow of nutri- 

 ment towards the central and external flowers : we 

 know, at least, than in irregular flowers, those nearest 

 to the axis are oftenest subject to peloria, and become 

 regular. I may add, as an instance of this, and of a 

 striking case of correlation, that I have recently ob- 

 served in some garden pelargoniums, that the central 

 flower of the truss often loses the patches of darker 

 colour in the two upper petals ; and that when this 

 occurs, the adherent nectary is quite aborted ; when 

 the colour is absent from only one of the two upper 

 petals, the nectary is only much shortened. 



With respect to the difference in the corolla of the 

 central and exterior flowers of a head or umbel, I do 

 not feel at all sure that C. C. Sprengel's idea that the 

 ray- florets serve to attract insects, whose agency is 

 highly advantageous in the fertilisation of plants of 

 these two orders, is so far-fetched, as it may at first 

 appear : and if it be advantageous, natural selection 

 may have come into play. But in regard to the differ- 

 ences both in the internal and external structure of the 

 seeds, which are not always correlated with any differ- 

 ences in the flowers, it seems impossible that they can 

 be in any way advantageous to the plant : yet in the 

 Umbelliferae these differences are of such apparent im- 

 portance — the seeds being in some cases, according to 

 Tausch, orthospermous in the exterior flowers and 

 coelospermous in the central flowers, — that the elder De 

 Candolle founded his main divisions of the order on 

 analogous differences. Hence we see that modifications 

 of structure, viewed by systematists as of high value, 

 may be wholly due to unknown laws of correlated 

 growth, and without being, as far as we can see, of the 

 slightest service to the species. 



We may often falsely attribute to correlation of 

 growth, structures which are common to whole groups 

 of species, and which in truth are simply due to in- 

 heritance ; for an ancient progenitor may have acquired 

 through natural selection some one modification in 



