440 GENEEAL RESULTS. Chap. Xlt 



their fertilisation ; and bearing in mind the prepo- 

 tency of pollen from a distinct individual over that 

 from the same individual, such plants will almost cer- 

 tainly have been crossed during many or all previous 

 generations. So it must be, owing merely to the 

 prepotency of foreign pollen, with cabbages and various 

 other plants, the varieties of which almost invariably 

 intercross when grown together. The same inference 

 may be drawn still more surely with respect to those 

 plants, such as of Eeseda and Eschscholtzia, which 

 are sterile with their own pollen, but fertile with 

 that from any other individual. These several plants 

 must therefore have been crossed during a long series 

 of previous generations, and the artificial crosses in my 

 experiments cannot have increased the vigour of the 

 offspring beyond that of their progenitors. Therefore 

 the difference between the self-fertilised and crossed 

 plants raised by me cannot be attributed to the supe- 

 riority of the crossed, but to the inferiority of the 

 self-fertilised seedlings, due to the injurious effects of 

 self-fertilisation. 



Notwithstanding the evil which many plants suffer 

 from self-fertilisation, they can be thus propagated 

 under favourable conditions for many generations, as 

 shown by some of my experiments, and more especially 

 by the survival during at least half a century of the 

 same varieties of the common pea and sweet-pea. The 

 same conclusion probably holds good with several other 

 exotic plants, which are never or most rarely cross- 

 fertilised in this country. But all these plants, as far 

 as they have been tried, profit greatly by a cross with 

 a fresh stock. Many species which bear small and 

 inconspicuous flowers are never, or most rarely, visited 

 by insects during the day; and Hermann Muller 

 infers that they must be always, or almost always, 



