1078. The power of hybridising 'appears to be far more common in plants than in animals; for while only a few animal mules are known, there is scarcely a genus of domesticated plants in which this effect cannot be produced by the assistance of man, in placing the pollen of one species upon the stigma of another. It is, however, in general, only between nearly allied species that this intercourse can take place ; those which are widely different in structure and constitution not being capable of any artificial union. Thus, the different species of strawberry, of certain tribes of Pelargonium, and of Cucurbitaceï¾µ, intermix with the greatest facility, there being a great accordance between them in general structure and constitution ; but no one has ever succeeded in compelling the pear to fertilise the apple, or the gooseberry the currant. And as species that are very dissimilar appear to have some natural impediment, which prevents their reciprocal fertilisation, so does this obstacle, of whatever nature it may be, present an insuperable bar to the intercourse of different genera. All the stories that are current as to the intermixture of oranges and pomegranates, of roses and black currants, and the like, may, therefore, be set down to pure invention.' (Lind. Introd. 1st ed. p. 302.)
