We have, however, on the other hand, overwhelming evidence that in many cases, among our domestic animals and cultivated plants, close interbreeding does produce bad results, and the apparent contradiction may perhaps be explained on the same general principles, and under similar limitations, as were found to be necessary in defining the value of intercrossing. It appears probable, then, that it is not interbreeding in itself that is hurtful, but interbreeding without rigid selection or some change of conditions. Under nature, as in the case of the Porto Santo rabbits, the rapid increase of these animals would in a very few years stock the island with a full population, and thereafter natural selection would act powerfully in the preservation only of the healthiest and the most fertile, and under these conditions no deterioration would occur. Among the aristocracy there has been a constant selection of beauty, which is generally synonymous with health, while any constitutional infertility has led to the extinction of the family. With domestic animals the selection practised is usually neither severe enough nor of the right kind. There is no natural struggle for existence, but certain points of form and colour characteristic of the breed are considered essential, and thus the most vigorous or the most fertile are not always those which are selected to continue the stock. In nature, too, the species always extends over a larger area and consists of much greater numbers, and thus a difference of constitution soon arises in different parts of the area, which is wanting in the limited numbers of pure bred domestic animals. From a consideration of these varied facts we conclude that an occasional disturbance of the organic equilibrium is what is essential to keep up the vigour and fertility of any organism, and that this disturbance may be equally well produced either by a cross between individuals of somewhat different constitutions, or by occasional slight changes in the conditions of life. Now plants which have great powers of dispersal enjoy a constant change of conditions, and can, therefore, exist permanently, or at all events, for very long periods, without intercrossing; while those which have limited powers of dispersal, and are restricted to a comparatively small and uniform area, need an occasional cross to keep up their fertility and general vigour. We should, therefore, expect that those groups of plants which are adapted both for cross-and self-fertilisation, which have showy flowers and possess great powers of seed-dispersal, would be the most abundant and most widely distributed; and this we find to be the case, the Compositae possessing all these characteristics in the highest degree, and being the most generally abundant group of plants with conspicuous flowers in all parts of the world.
How the Struggle for Existence Acts among Flowers.
Let us now consider what will be the action of the struggle for existence under the conditions we have seen to exist.
Everywhere and at all times some species of plants will be dominant and aggressive; while others will be diminishing in numbers, reduced to occupy a smaller area, and generally having a hard struggle to maintain themselves. Whenever a self-fertilising plant is thus reduced in numbers it will be in danger of extinction, because, being limited to a small area, it will suffer from the effects of too uniform conditions which will produce weakness and infertility. But while this change is in progress, any crosses between individuals of slightly different constitution will be beneficial, and all variations favouring either insect agency on the one hand, or wind-dispersal of pollen on the other, will lead to the production of a somewhat stronger and more fertile stock. Increased size or greater brilliancy of the flower, more abundant nectar, sweeter odour, or adaptations for more effectual cross-fertilisation would all be preserved, and thus would be initiated some form of specialisation for insect agency in cross-fertilisation; and in every different species so circumstanced the result would be different, depending as it would on many and complex combinations of variation of parts of the flower, and of the insect species which most abounded in the district.
Species thus favourably modified might begin a new era of development, and, while spreading over a somewhat wider area, give rise to new varieties or species, all adapted in various degrees and modes to secure cross-fertilisation by insect agency. But in course of ages some change of conditions might prove adverse. Either the insects required might diminish in numbers or be attracted by other competing flowers, or a change of climate might give the advantage to other more vigorous plants. Then self-fertilisation with greater means of dispersal might be more advantageous; the flowers might become smaller and more numerous; the seeds smaller and lighter so as to be more easily dispersed by the wind, while some of the special adaptations for insect fertilisation being useless would, by the absence of selection and by the law of economy of growth, be reduced to a rudimentary form. With these modifications the species might extend its range into new districts, thereby obtaining increased vigour by the change of conditions, as appears to have been the case with so many of the small flowered self-fertilised plants. Thus it might continue to exist for a long series of ages, till under other changes—geographical or biological—it might again suffer from competition or from other adverse circumstances, and be at length again confined to a limited area, or reduced to very scanty numbers.
But when this cycle of change had taken place, the species would be very different from the original form. The flower would have been at one time modified to favour the visits of insects and to secure cross-fertilisation by their aid, and when the need for this passed away, some portions of these structures would remain, though in a reduced or rudimentary condition. But when insect agency became of importance a second time, the new modifications would start from a different or more advanced basis, and thus a more complex result might be produced. Owing to the unequal rates at which the reduction of the various parts might occur, some amount of irregularity in the flower might arise, and on a second development towards insect cross-fertilisation this irregularity, if useful, might be increased by variation and selection.
The rapidity and comparative certainty with which such changes as are here supposed do really take place, are well shown by the great differences in floral structure, as regards the mode of fertilisation, in allied genera and species, and even in some cases in varieties of the same species. Thus in the Ranunculaceae we find the conspicuous part of the flower to be the petals in Ranunculus, the sepals in Helleborus, Anemone, etc., and the stamens in most species of Thalictrum. In all these we have a simple regular flower, but in Aquilegia it is made complex by the spurred petals, and in Delphinium and Aconitum it becomes quite irregular. In the more simple class self-fertilisation occurs freely, but it is prevented in the more complex flowers by the stamens maturing before the pistil. In the Caprifoliaceae we have small and regular greenish flowers, as in the moschatel (Adoxa); more conspicuous regular open flowers without honey, as in the elder (Sambucus); and tubular flowers increasing in length and irregularity, till in some, like our common honeysuckle, they are adapted for fertilisation by moths only, with abundant honey and delicious perfume to attract them. In the Scrophulariaceae we find open, almost regular flowers, as Veronica and Verbascum, fertilised by flies and bees, but also self-fertilised; Scrophularia adapted in form and colour to be fertilised by wasps; and the more complex and irregular flowers of Linaria, Rhinanthus, Melampyrum, Pedicularis, etc., mostly adapted to be fertilised by bees.
In the genera Geranium, Polygonum, Veronica, and several others there is a gradation of forms from large and bright to small and obscure coloured flowers, and in every case the former are adapted for insect fertilisation, often exclusively, while in the latter self-fertilisation constantly occurs. In the yellow rattle (Rhinanthus Crista-galli) there are two forms (which have been named major and minor), the larger and more conspicuous adapted to insect fertilisation only, the smaller capable of self-fertilisation; and two similar forms exist in the eyebright (Euphrasia officinalis). In both these cases there are special modifications in the length and curvature of the style as well as in the size and shape of the corolla; and the two forms are evidently becoming each adapted to special conditions, since in some districts the one, in other districts the other is most abundant.[159]
These examples show us that the kind of change suggested above is actually going on, and has presumably always been going on in nature throughout the long geological epochs during which the development of flowers has been progressing. The two great modes of gaining increased vigour and fertility—intercrossing and dispersal over wider areas—have been resorted to again and again, under the pressure of a constant struggle for existence and the need for adaptation to ever-changing conditions. During all the modifications that ensued, useless parts were reduced or suppressed, owing to the absence of selection and the principle of economy of growth; and thus at each fresh adaptation some rudiments of old structures were re-developed, but not unfrequently in a different form and for a distinct purpose.
The chief types of flowering plants have existed during the millions of ages of the whole tertiary period, and during this enormous lapse of time many of them may have been modified in the direction of insect fertilisation, and again into that of self-fertilisation, not once or twice only, but perhaps scores or even hundreds of times; and at each such modification a difference in the environment may have led to a distinct line of development. At one epoch the highest specialisation of structure in adaptation to a single species or group of insects may have saved a plant from extinction; while, at other times, the simplest mode of self-fertilisation, combined with greater powers of dispersal and a constitution capable of supporting diverse physical conditions, may have led to a similar result. With some groups the tendency seems to have been almost continuously to greater and greater specialisation, while with others a tendency to simplification and degradation has resulted in such plants as the grasses and sedges.