OBSERVATIONS UPON MULING AMONG PLANTS, 



169 



is no necessary element — or the contrary an hindrance, if by 

 chance or artificial means the pollen of the one species can be 

 procured when the stigma of the other is ready. In like manner 

 the duration of the time of growth, or diversity of habit and 

 persistence of foliage, is, to a certain extent, indifferent. The 

 fruticose Calceolaria unite with C. plantaginea ; the evergreen 

 Rhododendron with the deciduous Azalea, and Hyoseyamtts 

 niger with agrestis. 



In some cases, especially in those where hybridization is rare, 

 outward conditions, such as increased temperature, predispose 

 plants for hybrid impregnation, and cultivation in general is 

 favourable to this end, as it is to the production of deviations 

 from a normal condition. Varieties are usually far more dis- 

 posed to mix than the species from which they are derived, and 

 hence the great difficulty of keeping our most valuable vegetables 

 pure and genuine. Of all genera Calceolaria seems to present 

 the greatest tendency to hybridize; the pure species unite with 

 the utmost facility, and their hybrids are all fertile and disposed 

 to fresh admixture. 



As regards species, we know as little on what peculiarities the 

 power of producing hybrids depends as in genera. No differ- 

 ence is perceptible in structure, and species which will not 

 produce hybrids themselves, are sometimes capable of fecundat- 

 ing others ; some will hybridize with but one species, whilst 

 others show a greater extent of elective affinity. As, however, 

 those which hybridize with one species usually are susceptible 

 with regard to others, only in a different degree, it is possible 

 that species which have hitherto united with one only will be 

 found on further experiment to have a wider affinity. 



In questions relative to species, it is of course requisite to 

 have definite notions as to the terms species and variety, and the 

 stability of the species themselves. It is impossible, however, 

 to enter on the subject here. History is in favour of their 

 stability, and of the existence of certain typical forms from 

 which altered circumstances produce many deviations, all of 

 which, indeed, have a tendency to revert to the original from 

 whence they were derived. The facts which hybrid fecundation 

 exhibits are decidedly in favour of such stability, and afford in 

 doubtful cases the means of ascertaining specific distinctions. 

 For nearly related species united with some particular species 

 give rise to hybrids differing from one another more than the 

 pure species from which they sprang ; and in the case of such 

 species as Pentstemon ge?itianoides and coccineus (Hoffm.), 

 which differ principally in the colour of the flowers, their 

 absolute specific difference is shown from the fact that all 

 attempts to produce hybrids between them fail, which would not 



