The Different Forms of Flowers on Plants of the Same Species - Charles Robert Darwin (ebook reader screen TXT) 📗

number of seeds produced by the long-styled and short-styled flowers of the primrose, when both are illegitimately fertilised, is greater than that between the number produced under similar circumstances by the two forms of the cowslip. The long-styled flowers of the primrose when protected from the access of all insects, except such minute ones as Thrips, yield a considerable number of capsules containing on an average 19.2 seeds per capsule; whereas 18 plants of the long-styled cowslip similarly treated did not yield a single seed.
The primrose, as every one knows, flowers a little earlier in the spring than the cowslip, and inhabits slightly different stations and districts. The primrose generally grows on banks or in woods, whilst the cowslip is found in more open places. The geographical range of the two forms is different. Dr. Bromfield remarks that "the primrose is absent from all the interior region of northern Europe, where the cowslip is indigenous." (2/3. 'Phytologist' volume 3 page 694.) In Norway, however, both plants range to the same degree of north latitude. (2/4. H. Lecoq 'Geograph. Bot. de l'Europe' tome 8 1858 pages 141, 144. See also 'Annals and Magazine of Natural History' 9 1842 pages 156, 515. Also Boreau 'Flore du centre de la France' 1840 tome 2 page 376. With respect to the rarity of P. veris in western Scotland, see H.C. Watson 'Cybele Britannica' 2 page 293.)
The cowslip and primrose, when intercrossed, behave like distinct species, for they are far from being mutually fertile. Gartner crossed 27 flowers of P. vulgaris with pollen of P. veris, and obtained 16 capsules; but these did not contain any good seed. (2/5. 'Bastarderzeugung' 1849 page 721.) He also crossed 21 flowers of P. veris with pollen of P. vulgaris; and now he got only five capsules, containing seed in a still less perfect condition. Gartner knew nothing about heterostylism; and his complete failure may perhaps be accounted for by his having crossed together the same forms of the cowslip and primrose; for such crosses would have been of an illegitimate as well as of a hybrid nature, and this would have increased their sterility. My trials were rather more fortunate. Twenty-one flowers, consisting of both forms of the cowslip and primrose, were intercrossed legitimately, and yielded seven capsules (i.e. 33 per cent), containing on an average 42 seeds; some of these seeds, however, were so poor that they probably would not have germinated. Twenty-one flowers on the same cowslip and primrose plants were also intercrossed illegitimately, and they likewise yielded seven capsules (or 33 per cent), but these contained on an average only 13 good and bad seeds. I should, however, state that some of the above flowers of the primrose were fertilised with pollen from the polyanthus, which is certainly a variety of the cowslip, as may be inferred from the perfect fertility inter se of the crossed offspring from these two plants. (2/6. Mr. Scott has discussed the nature of the polyanthus ('Proceedings of the Linnean Society' 8 Botany 1864 page 103), and arrives at a different conclusion; but I do not think that his experiments were sufficiently numerous. The degree of infertility of a cross is liable to much fluctuation. Pollen from the cowslip at first appears rather more efficient on the primrose than that of the polyanthus; for 12 flowers of both forms of the primrose, fertilised legitimately and illegitimately with pollen of the cowslip gave five capsules, containing on an average 32.4 seeds; whilst 18 flowers similarly fertilised by polyanthus-pollen yielded only five capsules, containing only 22.6 seeds. On the other hand, the seeds produced by the polyanthus-pollen were much the finest of the whole lot, and were the only ones which germinated.) To show how sterile these hybrid unions were I may remind the reader that 90 per cent of the flowers of the primrose fertilised legitimately with primrose-pollen yielded capsules, containing on an average 66 seeds; and that 54 per cent of the flowers fertilised illegitimately yielded capsules containing on an average 35.5 seeds per capsule. The primrose, especially the short-styled form, when fertilised by the cowslip, is less sterile, as Gartner likewise observed, than is the cowslip when fertilised by the primrose. The above experiments also show that a cross between the same forms of the primrose and cowslip is much more sterile than that between different forms of these two species.
The seeds from the several foregoing crosses were sown, but none germinated except those from the short-styled primrose fertilised with pollen of the polyanthus; and these seeds were the finest of the whole lot. I thus raised six plants, and compared them with a group of wild oxlips which I had transplanted into my garden. One of these wild oxlips produced slightly larger flowers than the others, and this one was identical in every character (in foliage, flower- peduncle, and flowers) with my six plants, excepting that the flowers of the latter were tinged of a dingy red colour, from being descended from the polyanthus.
We thus see that the cowslip and primrose cannot be crossed either way except with considerable difficulty, that they differ conspicuously in external appearance, that they differ in various physiological characters, that they inhabit slightly different stations and range differently. Hence those botanists who rank these plants as varieties ought to be able to prove that they are not as well fixed in character as are most species; and the evidence in favour of such instability of character appears at first sight very strong. It rests, first, on statements made by several competent observers that they have raised cowslips, primroses, and oxlips from seeds of the same plant; and, secondly, on the frequent occurrence in a state of nature of plants presenting every intermediate gradation between the cowslip and primrose.
The first statement, however, is of little value; for, heterostylism not being formerly understood, the seed-bearing plants were in no instance protected from the visits of insects (2/7. One author states in the 'Phytologist' volume 3 page 703 that he covered with bell-glasses some cowslips, primroses, etc., on which he experimented. He specifies all the details of his experiment, but does not say that he artificially fertilised his plants; yet he obtained an abundance of seed, which is simply impossible. Hence there must have been some strange error in these experiments, which may be passed over as valueless.); and there would be almost as much risk of an isolated cowslip, or of several cowslips if consisting of the same form, being crossed by a neighbouring primrose and producing oxlips, as of one sex of a dioecious plant, under similar circumstances, being crossed by the opposite sex of an allied and neighbouring species. Mr. H.C. Watson, a critical and most careful observer, made many experiments by sowing the seeds of cowslips and of various kinds of oxlips, and arrived at the following conclusion, namely, "that seeds of a cowslip can produce cowslips and oxlips, and that seeds of an oxlip can produce cowslips, oxlips, and primroses." (2/8. 'Phytologist' 2 pages 217, 852; 3 page 43.) This conclusion harmonises perfectly with the view that in all cases, when such results have been obtained, the unprotected cowslips have been crossed by primroses, and the unprotected oxlips by either cowslips or primroses; for in this latter case we might expect, by the aid of reversion, which notoriously comes into powerful action with hybrids, that the two parent-forms in appearance pure, as well as many intermediate gradations, would be occasionally produced. Nevertheless the two following statements offer considerable difficulty. The Reverend Professor Henslow raised from seeds of a cowslip growing in his garden, various kinds of oxlips and one perfect primrose; but a statement in the same paper perhaps throws light on this anomalous result. (2/9. Loudon's 'Magazine of Natural History' 3 1830 page 409.) Professor Henslow had previously transplanted into his garden a cowslip, which completely changed its appearance during the following year, and now resembled an oxlip. Next year again it changed its character, and produced, in addition to the ordinary umbels, a few single- flowered scapes, bearing flowers somewhat smaller and more deeply coloured than those of the common primrose. From what I have myself observed with oxlips, I cannot doubt that this plant was an oxlip in a highly variable condition, almost like that of the famous Cytisus adami. This presumed oxlip was propagated by offsets, which were planted in different parts of the garden; and if Professor Henslow took by mistake seeds from one of these plants, especially if it had been crossed by a primrose, the result would be quite intelligible. Another case is still more difficult to understand: Dr. Herbert raised, from the seeds of a highly cultivated red cowslip, cowslips, oxlips of various kinds, and a primrose. (2/10. 'Transactions of the Horticultural Society' 4 page 19.) This case, if accurately recorded, which I much doubt, is explicable only on the improbable assumption that the red cowslip was not of pure parentage. With species and varieties of many kinds, when intercrossed, one is sometimes strongly prepotent over the other; and instances are known of a variety crossed by another, producing offspring which in certain characters, as in colour, hairiness, etc., have proved identical with the pollen-bearing parent, and quite dissimilar to the mother-plant (2/11. I have given instances in my work 'On the Variation of Animals and Plants under Domestication' chapter 15 2nd edition volume 2 page 69.); but I do not know of any instance of the offspring of a cross perfectly resembling, in a considerable number of important characters, the father alone. It is, therefore, very improbable that a pure cowslip crossed by a primrose should ever produce a primrose in appearance pure. Although the facts given by Dr. Herbert and Professor Henslow are difficult to explain, yet until it can be shown that a cowslip or a primrose, carefully protected from insects, will give birth to at least oxlips, the cases hitherto recorded have little weight in leading us to admit that the cowslip and primrose are varieties of one and the same species.
Negative evidence is of little value; but the following facts may be worth giving:--Some cowslips which had been transplanted from the fields into a shrubbery were again transplanted into highly manured land. In the following year they were protected from insects, artificially fertilised, and the seed thus procured was sown in a hotbed. The young plants were afterwards planted out, some in very rich soil, some in stiff poor clay, some in old peat, and some in pots in the greenhouse; so that these plants, 765 in number, as well as their parents, were subjected to diversified and unnatural treatment; but not one of them presented the least variation except in size--those in the peat attaining almost gigantic dimensions, and those in the clay being much dwarfed.
I do not, of course, doubt that cowslips exposed during SEVERAL successive generations to changed conditions would vary, and that this might occasionally occur in a state of nature. Moreover, from the law of analogical variation, the varieties of any one species of Primula would probably in some cases resemble other species of the genus. For instance I raised a red primrose from seed from a protected plant, and the flowers, though still resembling those of the primrose, were borne during one season in umbels on a long foot-stalk like that of a cowslip.
With regard to the second class of facts in support of the cowslip and primrose being ranked as mere varieties, namely, the well-ascertained existence in a state of nature of numerous linking forms (2/12. See an excellent article on this subject by Mr. H.C. Watson in the 'Phytologist' volume 3 page 43.):--If it can be shown that the common wild oxlip, which is intermediate in character between the cowslip and primrose, resembles in sterility and other essential respects a hybrid plant, and if it can further be shown that the oxlip,