114 Plant-Breeding 



fertilization. In short, he found, as he expressed it, that 

 " nature abhors perpetual self-fertilization." Some of his 

 particular results, although often quoted, will be useful in 

 fixing these facts in our minds. 



Darwin's experiments with morning-glories. Plants 

 from crossed seeds of morning-glory exceeded in height 

 those from self-fertilized seeds as 100 exceeds 76, in the 

 first generation. Some flowers from these plants were 

 self-pollinated and some were crossed, and in this second 

 generation the crossed plants were to the uncrossed as 

 100 is to 79 ; the operation was again repeated, and in the 

 third generation, the plant having been grown in mid- 

 winter, when none of them did well, 100 to 86 ; fifth 

 generation, 100 to 75 ; sixth generation, 100 to 72 ; seventh 

 generation, 100 to 81 ; eighth generation, 100 to 85 ; ninth 

 generation, 100 to 79 ; tenth generation, 100 to 54. The 

 average total gain in height of the crossed over the un- 

 crossed was as 100 to 77, or about 30 per cent. There 

 was a corresponding gain in fertility, or the number of 

 seeds and seed-pods produced. Yet, striking as the results 

 are, they were produced by simple crossing between plants 

 grown near together, and under what would ordinarily 

 be called uniform conditions. In order to determine the 

 influence of crossing with fresh stock, plants of the same 

 variety were obtained from another garden and these 

 were crossed with the ninth generation mentioned above. 

 The offspring of this cross exceeded those of the other 

 crossed plants as 100 exceeds 78, in height ; as 100 exceeds 

 57, in the number of seed-pods; and as 100 exceeds 51, 

 in the weight of the seed-pods. In other words, crosses 

 between fresh stock of the same variety were nearly 30 



