CHAP. XH. GENERAL RESULTS. 455 



most powerful of all the means for retaining some 

 degree of differentiation in the sexual elements, as 

 shown by the superiority in the later generations of the 

 intercrossed over the self-fertilised seedlings. Never- 

 theless, the continued intercrossing of plants thus 

 treated does tend to obliterate such differentiation, as 

 may be inferred from the lessened benefit derived from 

 intercrossing such plants, in comparison with that from 

 a cross with a fresh stock. It seems probable, as I may 

 add, that seeds have acquired their endless curious 

 adaptations for wide dissemination,* not only that the 

 seedlings should thus be enabled to find new and fitting 

 homes, but that the individuals which have been long 

 subjected to the same conditions should occasionally 

 intercross with a fresh stock. 



From the foregoing several considerations we may, 

 I think, conclude that in the above case of the Digitalis, 

 and even in that of plants which have grown for 

 thousands of generations in the same district, as must 

 often have occurred with species having a much 

 restricted range, we are apt to over-estimate the 

 degree to which the individuals have been subjected 

 to absolutely the same conditions. There is at least 

 no difficulty in believing that such plants have been 

 subjected to sufficiently distinct conditions to differ- 

 entiate their sexual elements ; for we know that a plant 

 propagated for some generations in another garden in 

 the same district serves as a fresh stock and has high 

 fertilising powers. The curious cases of plants which 

 can fertilise and be fertilised by any other individual 

 of the same species, but are altogether sterile with their 

 own pollen, become intelligible, if the view here pro- 

 pounded is correct, namely, that the individuals of the 



* See Prof Hildebrand's excellent treatise, 'Verb eitungsmittel der 

 Pflauzen,' 1873. 



