448 GENERAL RESULTS. Chap. XIL 



determined by the number of the pollen-tubes which 

 reach the ovules, and this will be governed by the re- 

 action between the pollen and the stigmatic secretion 

 or tissues; whereas the growth and constitutional vigour 

 of the offspring will be chiefly determined, not only by 

 the number of pollen-tubes reaching the ovules, but by 

 the nature of the reaction between the contents of the 

 pollen-grains and ovules. 



There are two other important conclusions which 

 may be deduced from my observations : firstly, that the 

 advantages of cross-fertilisation do not follow from 

 some mysterious virtue in the mere union of two 

 distinct individuals, but from such individuals having 

 been subjected during previous generations to dif- 

 ferent conditions, or to their having varied in a manner 

 commonly called spontaneous, so that in either case 

 their sexual elements have been in some degree differ- 

 entiated. And secondly, that the injury from self- 

 fertilisation follows from the want of such differentia- 

 tion in the sexual elements. These two propositions 

 are fully established by my experiments. Thus, when 

 plants of the Ipomoea and of the Mimulus, which had 

 been self-fertilised for the seven previous generations 

 and had been kept all the time under the same condi- 

 tions, were intercrossed one with another, the offspring 

 did not profit in the least by the cross. Mimulus 

 offers another instructive case, showing that the 

 benefit of a cross depends on the previous treatment 

 of the progenitors : plants which had been self-fer- 

 tilised for the eight previous generations were crossed 

 with plants which had been intercrossed for the same 

 number of generations, all having been kept under 

 the same conditions as far as possible ; seedlings from 

 this cross were grown in competition with others 



