264 SUMMARY OF MEASUREMENTS. CHAP. VIL 



for three generations were crossed with pollen from a 

 fresh stock, and their offspring were grown in compe- 

 tition with plants of the fourth self-fertilised genera- 

 tion. The crossed plants thus obtained were to the 

 self-fertilised in height as 100 to 81, and in fertility 

 (both lots being left to be naturally fertilised by 

 insects) as 100 to 33. 



These same crossed plants were also to the off- 

 spring from the plants of the third self-fertilised gene- 

 ration crossed by the intercrossed plants of the cor- 

 responding generation, in height as 100 to 85, and in 

 fertility as 100 to 45. 



We thus see what a great advantage the offspring 

 from a cross with a fresh stock had, not only over the 

 self-fertilised plants of the fourth generation, but 

 over the offspring from the self-fertilised plants of the 

 third generation, when crossed by the intercrossed 

 plants of the old stock. 



(7.) Pisum sativum. It has been shown under the 

 head of this species, that the several varieties in this 

 country almost invariably fertilise themselves, owing 

 to insects rarely visiting the flowers; and as the 

 plants have been long cultivated under nearly similar 

 conditions, we can understand why a cross between 

 two individuals of the same variety does not do the 

 least good to the offspring either in height or fertility. 

 This case is almost exactly parallel with that of 

 Mimulus, or that of the Ipomosa named Hero; for 

 in these two instances, crossing plants which had been 

 self-fertilised for seven generations did not at all 

 benefit the offspring. On the other hand, a cross 

 between two varieties of the pea causes a marked 

 superiority in the growth and vigour of the offspring, 

 over the self-fertilised plants of the same varieties, 

 as shown by two excellent observers. From my own 



