

146 LAWS OF VARIATION. Chap. V. 



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 coe- 

 lospermous in the central flowers, — that the elder De 

 Candolle founded his main divisions of the order on 

 analogous differences. Hence we see that modifica- 

 tions 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 inheritance ; 

 for an ancient progenitor may have acquired through 

 natural selection some one modification in structure, 

 and, after thousands of generations, some other and in- 

 dependent modification ; and these two modifications, 

 having been transmitted to a whole group of descendants 

 with diverse habits, would naturally be thought to be 

 correlated in some necessary manner. So, again, I do 

 not doubt that some apparent correlations, occurring 

 throughout whole orders, are entirely due to the manner 

 . alone in which natural selection can act. For instance, 

 ' Alph. De Candolle has remarked that winged seeds are 

 \ ^ never found in fruits which do not open : I should ex- 

 plain the rule by the fact that seeds could not gradually 

 become winged through natural selection, except in fruits 

 which opened ; so that the individual plants producing 



