166 OBSERVATIONS UPON MULING AMONG PLANTS. 



the next chapter, relating to the aptness of plants, whether as 

 regards families and genera, or species, for hybrid fecundation. 



The question then arises whether there are any outward signs 

 by which this aptness may be discovered. Experience shows 

 that a great, perhaps the greater portion of plants are not sus- 

 ceptible of hybridization. Out of 700 species submitted to 

 nearly 10,000 distinct sets of experiments, only 250 true hybrids 

 were raised. Allowing the possibility of repeated experiments 

 proving that union is possible in some cases where it has not yet 

 been obtained, the result is sufficiently striking, showing, espe- 

 cially when taken in conjunction with the large number of 

 failures, where success has in some cases been obtained, not only 

 that the least portion of the vegetable kingdom is capable of 

 hybrid fecundation, but that as a general rule it is a forcing of 

 nature. 



In the larger portion of plants an harmonious concurrence of 

 time and place favours the production of pure species, though 

 this is sometimes disturbed, and in consequence hybridization 

 becomes possible ; and in any case a certain harmony of either 

 sexual element must exist to consummate a real hybrid union. 

 In this concurrence consists the fitness for union of two hetero- 

 geneous species. For this purpose a mutual attraction between 

 the stigma and the pollen must exist, the attraction being greater 

 for its own pollen, and in proportion to the degree of attraction 

 for heterogeneous pollen will be the fitness for union. But there 

 is no outer sign by which the degree of such attraction can be 

 known — it can be ascertained only from experience. It appears 

 probable, too, that this attraction does not exist in an equal 

 degree in every flower. At least, the causes which prevent suc- 

 cess are often perfectly inappreciable. It resides evidently in 

 some peculiar constitution which very possibly may accompany 

 peculiar organization. Unions of plants belonging to different 

 natural orders may be pronounced impossible, alleged instances 

 of such union being merely cases in which the pollen has been 

 perfectly inactive, and the supposed hybrid fecundation simply an 

 impregnation of the stigma with its own pollen. The author 

 has attempted to produce hybrids in twenty-nine natural orders, 

 and nine other orders are recorded as having given positive 

 results. In ten families out of the twenty-nine no real hybrids 

 were produced. Of these natural orders only a few genera were 

 submitted to experiment ; but of these a great part gave no 

 hybrids, and even the number of species which did so was very 

 limited. The most natural families, such as Graminece, Umbel- 

 latce, &c, are precisely those which, whether in a wild state or 

 under cultivation, seem least favourable to the production of 

 hybrids. Many hybrids are supposed to exist amongst C&rnpo- 



