FERTILIZATION IN THE VEGETABLE KINGDOM. 221 



same variety but from a distant garden. The resulting seed- 

 lings showed the benefit of the fresh stock remarkably, being 

 as much superior in vigor to those of the tenth intercrossed 

 generation as the latter were to the self-fertilized plants of a 

 corresponding generation. In height they were as 100 to 78, 

 over the ordinary intercrossed; 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 pro- 

 genitors having been subjected during several generations to 

 slightly different conditions, or to what we call in our igno- 

 rance spontaneous variation." 



The greater constitutional vigor of the crossed plants of 

 Ipomaea was manifested in other ways than their rate or 

 amount of growth : they better endured exposure to a low 

 temperature or sudden changes of temperature ; they blos- 

 somed earlier ; and they were more fertile. The difference 

 in fertility varied greatly in degree (the extremes in dif- 

 ferent experiments and in different generations being 100 to 

 99 and 100 to 26), but was always sustained. Also, "the 

 impaired fertility of the self -fertilized plants was shown in 

 another way, namely, by their anthers being smaller than 

 those in the flowers on the crossed plants. 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 im- 

 paired fertility of the self-fertilized plants of the eighth gen- 

 eration 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, belong- 

 ing to fifty-two genera, and to thirty great natural families, 

 the species being natives of all parts of the world. The re- 

 sults — the details and discussion of which occupy the bulk of 

 this volume — vary greatly, some plants making a better and 



