CHAP. XII. GENERAL RESULTS. 443 



Plants of Ipomoea were intercrossed for nine successive 

 generations; they were then again intercrossed, and 

 at the same time crossed with a plant of a fresh stock, 

 that is, one brought from another garden ; and the 

 offspring of this latter cross were to the intercrossed 

 plants of the tenth generation in height as 100 to 78, 

 and in fertility as 100 to 51. An analogous experi- 

 ment with Eschscholtzia gave a similar result, as far 

 as fertility was concerned. In neither of these cases 

 were any of the plants the product of self-fertilisation. 

 Plants of Dianthus were self- fertilised for three genera- 

 tions, and this no doubt was injurious ; but when 

 these plants were fertilised by a fresh stock and by 

 intercrossed plants of the same stock, there was a great 

 difference in fertility between the two sets of seedlings 

 and some difference in their height. Petunia offers 

 a nearly parallel case. With various other plants, the 

 wonderful effects of a cross with a fresh stock may 

 be seen in Table C. Several accounts have also been 

 published* of the extraordinary growth of seedlings 

 from a cross between two varieties of the same species, 

 some of which are known never to fertilise themselves ; 

 so that here neither self -fertilisation nor relationship 

 even in a remote degree can have come into play. We 

 may therefore conclude that the above two propositions 

 are true, that cross-fertilisation is generally beneficial 

 and self-fertilisation often injurious to the offspring. 



That certain plants, for instance, Viola tricolor, 

 Digitalis purpurea, Sarothamnus scoparius, Cyclamen 

 persicum, &c., which have been naturally cross-fertilised 

 for many or all previous generations, should suffer to 

 an extreme degree from a single act of self-fertilisation 

 is an astonishing fact. The evil does not depend in 



* See 'Variation under Domestication,' ch. xix. 2nd edit. vol. ii. ix 

 159. 



