Cross-Breeding and Hybridizing. n 



cross-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. Plants from crossed 

 seeds of morning-glory exceeded in height those from self-fertilized seeds as 

 ioo exceeds 76 in the first generation. Some flowers upon these plants were 

 self-pollinated and some were crossed, and in this sjecond generation the 

 crossed plants were to the uncrossed as 100 is to 79 ; the operation was again 

 repeated, and in the third generation the figures stand 100 to 68 ; fourth 

 generation, the plants having been grown in midwinter 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 genera- 

 tion, 100 to 79; tenth generation, 100 to 54. The average total gain in 

 height of the crossed over the uncrossed 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 these results are, they were 

 produced by simply 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 genera- 

 tion 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 from fresh stock of the same variety were nearly 30 per 

 cent, more vigorous than crosses between plants grown side by side for some 

 time, and over 44 per cent, more vigorous than plants from self-fertilized 

 seeds. It is evident, from all these figures, that nature desires crosses be- 

 tween plants, and, if possible, between plants grown under somewhat differ- 

 ent conditions. All the results are exceedingly interesting and important, 

 and there is every reason to believe that, as a rule, similar results can be ob- 

 tained with all plants. 



Darwin extended his investigations to many plants, only a few of which need 

 be discussed here. Cabbage gave pronounced results. Crossed plants were 

 to self-fertilized plants in weight as 100 to 37. A cross was now made be- 

 tween these crossed plants and a plant of the same variety from another 

 garden, and the difference in weight of the resulting offspring was the dif- 

 ference between 100 and 22, showing a gain of over 350 per cent, due to a 

 cross with fresh stock. Crossed lettuce-plants exceeded uncrossed in height 

 as 100 exceeds 82. Buckwheat gave an increase in weight of seeds as 100 to 

 82, and in height of plant as 100 to 69. Beets gave an increase in height 

 represented by 100 and 87. Maize, when full-grown, from crossed and un- 



