Several distinguished botanists and good practical judges believe that long- continued propagation by cuttings, runners, tubers, bulbs, etc., independently of any excessive development of these parts, is the cause of many plants failing to produce flowers, or producing only barren flowers,—it is as if they had lost the habit of sexual generation. (18/109. Godron 'De l'Espece' tome 2 page 106; Herbert on Crocus in 'Journal of Hort. Soc.' volume 1 1846 page 254: Dr. Wight, from what he has seen in India, believes in this view; 'Madras Journal of Lit. and Science' volume 4 1836 page 61.) That many plants when thus propagated are sterile there can be no doubt, but as to whether the long continuance of this form of propagation is the actual cause of their sterility, I will not venture, from the want of sufficient evidence, to express an opinion.
That plants may be propagated for long periods by buds, without the aid of sexual generation, we may safely infer from this being the case with many plants which must have long survived in a state of nature. As I have had occasion before to allude to this subject, I will here give such cases as I have collected. Many alpine plants ascend mountains beyond the height at which they can produce seed. (18/110. Wahlenberg specifies eight species in this state on the Lapland Alps: see Appendix to Linnaeus 'Tour in Lapland' translated by Sir J.E. Smith volume 2 pages 274-280.) Certain species of Poa and Festuca, when growing on mountain-pastures, propagate themselves, as I hear from Mr. Bentham, almost exclusively by bulblets. Kalm gives a more curious instance (18/111. 'Travels in North America' English translation volume 3 page 175.) of several American trees, which grow so plentifully in marshes or in thick woods, that they are certainly well adapted for these stations, yet scarcely ever produce seeds; but when accidentally growing on the outside of the marsh or wood, are loaded with seed. The common ivy is found in Northern Sweden and Russia, but flowers and fruits only in the southern provinces. The Acorus calamus extends over a large portion of the globe, but so rarely perfects fruit that this has been seen only by a few botanists; according to Caspary, all its pollen-grains are in a worthless condition. (18/112. With respect to the ivy and Acorus see Dr. Broomfield in the 'Phytologist' volume 3 page 376. Also Lindley and Vaucher on the Acorus and see Caspary as below.) The Hypericum calycinum, which propagates itself so freely in our shrubberies by rhizomes, and is naturalised in Ireland, blossoms profusely, but rarely sets any seed, and this only during certain years; nor did it set any when fertilised in my garden by pollen from plants growing at a distance. The Lysimachia nummularia, which is furnished with long runners, so seldom produces seed-capsules, that Prof. Decaisne (18/113. 'Annal. des Sc. Nat.' 3rd series Zool. tome 4 page 280. Prof. Decaisne refers also to analogous cases with mosses and lichens near Paris.), who has especially attended to this plant, has never seen it in fruit. The Carex rigida often fails to perfect its seed in Scotland, Lapland, Greenland, Germany, and New Hampshire in the United States. (18/114. Mr. Tuckermann in Silliman's 'American Journal of Science' volume 65 page 1.) The periwinkle (Vinca minor), which spreads largely by runners, is said scarcely ever to produce fruit in England (18/115. Sir J.E. Smith 'English Flora' volume 1 page 339.); but this plant requires insect-aid for its fertilisation, and the proper insects may be absent or rare. The Jussiaea grandiflora has become naturalised in Southern France, and has spread by its rhizomes so extensively as to impede the navigation of the waters, but never produces fertile seed. (18/116. G. Planchon 'Flora de Montpellier' 1864 page 20.) The horse-radish (Cochleria armoracia) spreads pertinaciously and is naturalised in various parts of Europe; though it bears flowers, these rarely produce capsules: Professor Caspary informs me that he has watched this plant since 1851, but has never seen its fruit; 65 per cent of its pollen-grains are bad. The common Ranunculus ficaria rarely bears seed in England, France, or Switzerland; but in 1863 I observed seeds on several plants growing near my house. (18/117. On the non-production of seeds in England see Mr. Crocker in 'Gardener's Weekly Magazine' 1852 page 70; Vaucher 'Hist. Phys. Plantes d'Europe' tome 1 page 33; Lecoq 'Geograph. Bot. d'Europe' tome 4 page 466; Dr. D. Clos in 'Annal. des Sc. Nat.' 3rd series Bot. tome 17 1852 page 129: this latter author refers to other analogous cases. See more especially on this plant and on other allied cases Prof. Caspary "Die Nuphar" 'Abhand. Naturw. Gesellsch. zu Halle' b. 11 1870 page 40, 78.) Other cases analogous with the foregoing could be given; for instance, some kinds of mosses and lichens have never been seen to fructify in France.
Some of these endemic and naturalised plants are probably rendered sterile from excessive multiplication by buds, and their consequent incapacity to produce and nourish seed. But the sterility of others more probably depends on the peculiar conditions under which they live, as in the case of the ivy in the northern part of Europe, and of the trees in the swamps of the United States; yet these plants must be in some respects eminently well adapted for the stations which they occupy, for they hold their places against a host of competitors.]
Finally, the high degree of sterility which often accompanies the doubling of flowers, or an excessive development of fruit, seldom supervenes at once. An incipient tendency is observed, and continued selection completes the result. The view which seems the most probable, and which connects together all the foregoing facts and brings them within our present subject, is, that changed and unnatural conditions of life first give a tendency to sterility; and in consequence of this, the organs of reproduction being no longer able fully to perform their proper functions, a supply of organised matter, not required for the development of the seed, flows either into these organs and renders them foliaceous, or into the fruit, stems, tubers, etc., increasing their size and succulency. But it is probable that there exists, independently of any incipient sterility, an antagonism between the two forms of reproduction, namely, by seed and buds, when either is carried to an extreme degree. That incipient sterility plays an important part in the doubling of flowers, and in the other cases just specified, I infer chiefly from the following facts. When fertility is lost from a wholly different cause, namely, from hybridism, there is a strong tendency, as Gartner (18/118. 'Bastarderzeugung' s. 565. Kolreuter 'Dritte Fortsetzung' s. 73, 87, 119) also shows that when two species, one single and the other double, are crossed, the hybrids are apt to be extremely double.) affirms, for flowers to become double, and this tendency is inherited. Moreover, it is notorious that with hybrids the male organs become sterile before the female organs, and with double flowers the stamens first become foliaceous. This latter fact is well shown by the male flowers of dioecious plants, which, according to Gallesio (18/119. 'Teoria della Riproduzione Veg.' 1816 page 73.) first become double. Again, Gartner (18/120. 'Bastarderzeugung' s. 573.) often insists that the flowers of even utterly sterile hybrids, which do not produce any seed, generally yield perfect capsules or fruit,—a fact which has likewise been repeatedly observed by Naudin with the Cucurbitaceae; so that the production of fruit by plants rendered sterile through any cause is intelligible. Kolreuter has also expressed his unbounded astonishment at the size and development of the tubers in certain hybrids; and all experimentalists (18/121. Ibid s. 527.) have remarked on the strong tendency in hybrids to increase by roots, runners, and suckers. Seeing that hybrid plants, which from their nature are more or less sterile, thus tend to produce double flowers; that they have the parts including the seed, that is the fruit, perfectly developed, even when containing no seed; that they sometimes yield gigantic roots; that they almost invariably tend to increase largely by suckers and other such means;—seeing this, and knowing, from the many facts given in the earlier parts of this chapter, that almost all organic beings when exposed to unnatural conditions tend to become more or less sterile, it seems much the most probable view that with cultivated plants sterility is the exciting cause, and double flowers, rich seedless fruit, and in some cases largely-developed organs of vegetation, etc., are the indirect results—these results having been in most cases largely increased through continued selection by man.
CHAPTER 2.XIX.
SUMMARY OF THE FOUR LAST CHAPTERS, WITH REMARKS ON HYBRIDISM.
ON THE EFFECTS OF CROSSING. THE INFLUENCE OF DOMESTICATION ON FERTILITY. CLOSE INTERBREEDING. GOOD AND EVIL RESULTS FROM CHANGED CONDITIONS OF LIFE. VARIETIES WHEN CROSSED NOT INVARIABLY FERTILE. ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND VARIETIES. CONCLUSIONS WITH RESPECT TO HYBRIDISM. LIGHT THROWN ON HYBRIDISM BY THE ILLEGITIMATE PROGENY OF HETEROSTYLED PLANTS. STERILITY OF CROSSED SPECIES DUE TO DIFFERENCES CONFINED TO THE REPRODUCTIVE SYSTEM. NOT ACCUMULATED THROUGH NATURAL SELECTION. REASONS WHY DOMESTIC VARIETIES ARE NOT MUTUALLY STERILE. TOO MUCH STRESS HAS BEEN LAID ON THE DIFFERENCE IN FERTILITY BETWEEN CROSSED SPECIES AND CROSSED VARIETIES. CONCLUSION.
It was shown in the fifteenth chapter that when individuals of the same variety, or even of a distinct variety, are allowed freely to intercross, uniformity of character is ultimately acquired. Some few characters, however, are incapable of fusion, but these are unimportant, as they are often of a semi-monstrous nature, and have suddenly appeared. Hence, to preserve our domesticated breeds true, or to improve them by methodical selection, it is obviously necessary that they should be kept separate. Nevertheless, a whole body of individuals may be slowly modified, through unconscious selection, as we shall see in a future chapter, without separating them into distinct lots. Domestic races have often been intentionally modified by one or two crosses, made with some allied race, and occasionally even by repeated crosses with very distinct races; but in almost all such cases, long-continued and careful selection has been absolutely necessary, owing to the excessive variability of the crossed offspring, due to the principle of reversion. In a few instances, however, mongrels have retained a uniform character from their first production.
When two varieties are allowed to cross freely, and one is much more numerous than the other, the former will ultimately absorb the latter. Should both varieties exist in nearly equal numbers, it is probable that a considerable period would elapse before the acquirement of a uniform character; and the character ultimately acquired would largely depend on prepotency of transmission and on the conditions of life; for the nature of these conditions would generally favour one variety more than another, so that a kind of natural selection would come into play. Unless the crossed offspring were slaughtered by man without the least discrimination, some degree of unmethodical selection would likewise come into action. From these several considerations we may infer, that when two or more closely allied species first came into the possession of the same tribe, their crossing will not have influenced, in so great a degree as has often been supposed, the character of the offspring in future times; although in some cases it probably has had a considerable effect.
Domestication, as a general rule, increases the prolificness of animals and plants. It eliminates the tendency to sterility which is common to species when first taken from a state of nature and crossed. On this latter head we have no direct evidence; but as our races of dogs, cattle, pigs etc., are almost certainly descended from aboriginally distinct stocks, and as these races are now fully fertile together, or at least incomparably more fertile than most species when crossed, we may with entire confidence accept this conclusion.