128 Darwin on the Effects of Cross- and Self- Fertilization. 



ments led to the same conclusion, namely, that there was no 

 particular benefit from cross-fertilization on the same plant. 

 In the other case, the cross was made not only between the 

 flowers of distinct plants, but between those from different 

 sources, and which had presumably grown under somewhat 

 different conditions. For instance, several flowers of the ninth 

 generation of crossed plants of Lpomoza were crossed w i 

 taken from the same variety, but from a distant garden. The 

 resulting seedlings showed the benefit of the fresh stock remark- 

 ably, being as much superior in vigor to those of I 

 inn r-crossed generation as the latter were to the self-' 

 plants of the corresponding generation. In height they were 

 as 100 to 78, over the ordinary inter-crossed; and in fertility, 

 as 100 to 51. Indeed, Mr. Darwin's main conclusion from all 

 his observations is, "that the mere act of crossing by itself does 

 no good. The good depends on the individuals which are 

 crossed differing slightly in constitution, owing to their progen- 

 ia been subjected during several generations to slightly 

 different conditions, or to what we call in our ignorance sponta- 



The greater constitutional vigor of the crossed plants c 

 Ipomcea was manifested in other ways than their rate or amour 

 wth; they better endured exposure to a low temperatui 



or sudden changes of temperature ; they blossomed earlie 

 they were more fertile. TJa ility varied l 



in degree (the extremes in different experiments and in differ- 



»ns being 100 to 99 and 100 to 26), but was always 



sustained. Also, "the impaired fertility of the sell- 

 plants was shown in another way, namely, by their anthers 

 being smaller than those in the flowers on th< < ro-<. 1 p! mts. 

 This was first observed in the seventh generation, but may 

 have occurred earlier. .... The quantity of pollen contained 

 in one of the self-fertilized was, as far as could be judged by 

 the eye, about half of that contained in one from a crossed 

 plant. The impaired fertility of the self-fertilized plants of the 

 eighth generation was also shown in another manner, which 

 may often be observed in hybrids — namely, by the first-formed 

 flowers being sterile." 



Similar experiments were made, but not carried to the same 

 extent, upon fifty -seven other species of plants, belonging to 

 fifty-two genera, and to thirty great natural families, the species 

 being natives of various parts of the world. The results— the 

 details and discussion of which occupy the bulk of this volume 

 — vary greatly, some plants making a better, and others a less 

 good showing for the advantage of cross-fertilizing, and this 

 advantage manifesting itself in different ways, some in vigor or 

 amount of growth, some in hardiness, most in fertility ; but 



