1080. The cause of the sterility of mule plants, Dr. Lindley observes, 'is at present entirely unknown. Sometimes, indeed, a deficiency of pollun may be assigned ; but in many cases there is no perceptible difference in the healthiness of structure of the fertilising organs of a mule plant, and of its parents. I know of no person who has attempted to prove this by comparative anatomical observations, except Professor Henslow of Cambridge; who in an excellent paper upon a hybrid Digitalis, investigated anatomically the conditions of the stamens and pistillum, both of his hybrid and its two parents, with great care and skill. The result of his inquiry was, that no appreciable difference could be detected. Although the power of creating mule plants, that are fertile for two or three generations, incontestably exists, yet in wild nature hybrid varieties are far from common; or, at least, there are few well attested instances of the fact. Among the most remarkable cases are, the Cistus Ledon constantly produced between C. monspessulanus and laurifolius and C'istus longifolius between C. monspessulanus and populifolius, in the wood of Fontfroide, near Narbonne, mentioned by Mr. Bentham. Again, the same acute botanist ascertained that Saxifraga lutcopurpurea of Lapeyrouse, and S. ambigua of De Candolle, are only wild accidental hybrids between S. aretioides and calyciflora : they are only found when the two parents grow together; but there they form a suite of intermediate states between the two. Gentians, having a similar origin, have also been remarked upon the mountains of Europe. It is difficult not to believe that a great number of the reputed species of Salix, Rosa, Rubus, and other intricate genera, have also had a hybrid origin; but I am not aware that there is at present any positive proof of this.' (Lind. Introd., 1st ed. p. 304.)
