SYSTEMS OF BREEDING 



621 



morning-glory (Ipomcea purpured)! This species was bred both 

 crossed and self -fertilized for ten generations. In every genera- 

 tion the crossed forms were larger than the self -fertilized, the 

 average being as 100 is to 77. Not only that, but they were 

 clearly the more productive. The species, therefore, is one that 

 on the whole is extremely sensitive to inbreeding. Let us, how- 

 ever, analyze the details of the experiment and observe how it 

 fares with individual plants. 



Darwin's plan was to put the cross and the inbred seeds into 

 moist sand at the same time, and then to pair them off in the 

 order of their germination. That is, the first cross-bred seed up 

 would be paired with the first inbred seed up as a competitor, 2 

 the two being planted on opposite sides of the same pot ; the 

 second would be paired with the second, the third with the third, 

 and so on. 3 



The following table, reporting the first generation, shows how 

 the results appeared at first. HIGHTS QF CROSS . BRED AND IN _ 



The average of these six BRED STOCK) _ FIRST GENERATION* 

 pairs is 86 inches for the 

 cross-bred and 65.6 for the 

 self -fertilized, an initial differ- 

 ence of approximately 20 

 inches, which on the whole 

 did not greatly change during 

 the ten generations of the ex- 

 periment. 



It will be noted that in this 

 table every inbred plant is 

 inferior to its cross-bred mate ; 

 not only that, but no inbred 

 individual of the series is as good as the poorest cross-bred 

 reported. 



1 Reported in full in Darwin's Cross and Self Fertilization, etc., chap, ii, 

 pp. 28-62. 



2 If, however, a seed germinated long before a corresponding mate appeared, 

 it was thrown away, the aim being to mate seedlings that germinated exactly 

 together, giving an even start. 



3 Darwin, Cross and Self Fertilization, etc., pp. n, 12. 



4 Ibid. p. 29. 



