was until now no clear and direct evidence that cross-fertiliza- 

 tion in the vegetable kingdom did re-invigorate. Indeed, the 

 contrary might be inferred from the long and seemingly indefi- 

 nite perpetuation of bud-propagating varieties, which have no 

 n at all. But the inference from this is not as cogent 

 as would at first appear. For, although bud -propagation is, 

 we think, to be considered as the extreme of close-breeding, 

 yet in it the amount of material contributed by parent to off- 

 spring is usually vastly more than in sexual reproduction ; 

 and, accordingly, the diminution to an injurious degree of any 

 inherited quality or essence might be correspondingly remote. 

 Yet. as sexual reproduction may be and often must be much 

 closer in plants than it can be in most animals, the ill effects of 

 ization, or the good of cross-fertilization, might the 

 sooner be noticeable. Mr. Darwin arranged a course of ex- 



Eeriments to test this question, prosecuted it as to some species 

 3r eleven years; and the main object of this volume is to set 

 forth the results. 



Ipomcea purpurea, the common Morning Glory of our gardens, 

 was the leading subject. The flowers of" this species self-fertil- 

 ize, but must also be habitually cross-fertilized, as they are 

 visited freely by humble-bees and other insects. Ten flowers 

 of a plant in a greenhouse were fertilized with their own pollen ; 

 ten others were crossed with pollen from a different plant. The 

 seeds from both were gathered, allowed to germinate on damp 

 sand, and as often as pairs germinated at the same time the two 

 were planted on opposite sides of the same pot, the soil in which 

 was well mixed, so as to be uniform in composition. "The 

 plants on the two sides were always watered at the same time 

 and as equally as possible, and even if this had not been done 

 the water would have spread almost equally to both sides, as 

 the pots were not large. The crossed and self-fertilized plants 

 were separated by a superficial partition, which was always 

 kept directed towards the chief source of the light, so that the 

 plants on both sides were equally illuminated." Five pairs 

 were thus planted in two pots, and all the remaining seeds, 

 whether or not in a state of germination, were planted on the 

 opposite sides of a third pot, so that the plants were crowded 

 and exposed to a very severe competition. Rods of equal 

 diameter were given to all the plants to twine up, and as soon 

 as one of each pair had reached the summit, both were measured. 

 But a single rod was furnished to each side of the crowded pot, 

 and only the tallest plant on each side was measured. This 

 was followed up, for ten generations; the close far! 

 being always self-fertilization, i. e., by pollen to stigma of the 

 same flower; the crossing between individuals in' successive 

 generations of this same stock, except in special instances. 



