Page:Natural History Review (1861).djvu/152

140 may often be observed in great numbers for hundreds of yards along a roadside, all of one sex, evidently all from suckers, originating, perhaps, many years back in a single individual. In like manner, an individual bramble will, in the course of years, spread through a whole wood; a fragment of coltsfoot or couch grass infest large fields; or Elodea Canadensis fill our canals, though not a single seedling be raised.

One of the greatest difficulties in arriving at a just conclusion as to the value to be attached to intermediate forms, is owing to the doubts which still hang over the question of hybridity. The existence of hybrids in the vegetable kingdom, less perfect in their nature than true species, analogous to the mule among animals, has at all times been a popular notion; and wild plants, having some resemblance to cultivated or useful ones, but less perfect in respect of the qualities sought from them, have in most countries been stigmatised as bastards. Linnæus corrected many of these popular errors which had crept into the scientific nomenclature of the day; but he still gave his sanction to the idea of the hybrid origin of certain species, by adopting the term as the specific name in certain cases, without, however, probably having given the matter much consideration. Since his time it has been shown that his Chelidonium hybridum, Vioia hybrida, Campanula hybrida, Chenopodium hybridum, &c., are genuine, substantive species; and the existence of hybrids in a state of nature has been denied by several botanists, and admitted only with great reservations by some even of the most distinguished ones of the present day. Others, on the contrary, of our most acute observers, having acquired convincing evidence of natural hybridity in a few cases, have generalized their conclusions; they have supposed natural hybrids to be of constant and frequent occurrence; and they have ascribed to this cause alone the majority of variations from the supposed typical forms of species, or even attributed to original hybridisations the multitude of nearly allied, but constant species, in several of the largest genera.

That wild hybrids do exist, I had already convincing evidence from personal observation during the years 1825 and 1826, when my attention was specially directed to the search after them in the Pyrenees and the South of France; and the proofs brought forward by other observers are not to be resisted. But the cases are very few, and it requires great caution before we can attribute to this cause the appearance of individuals of a species showing some approach in their characters to some other species. In Western Europe, there are but six genera in which I have myself been able to collect satisfactory proofs of natural hybrids, viz., Cistus (including Helianthemum), Geum, Saxifraga, Gentiana, Verbascum, and Digitalis. We are also bound to admit on the authority of other observers, at least four more, viz., Epilobium, Carduus (including Cirsium), Salix, and Narcissus, and perhaps also Centaurea, Erica, Rumex, and Polygonum. The supposed hybrids in Viola, Medicago, Primula, if cross-breeds at all, are probably between varieties of one species, not between two species. The cases adduced in Serapias, Aceras, and Orchis, require much farther investigation, especially now