A first book in organic evolution

by D. Kerfoot Shute

Among mammals the males win the females much more through fighting than through the display of charms. In the breeding season the most timid male animals, which are not even supplied with any special weapons for fighting, engage in the most desperate conflicts. Two male hares have been seen to fight until one was killed. Those male mammals which are provided with special weapons for fighting enter into the fiercest and most deadly conflicts. The wild male elephant, during the period of love, is one of the fiercest fighters in the world. Lions engage in terrible battles, and a young lion dare not approach an old one.

Male seals fight most desperately during the breeding season, using both their claws and teeth. The conquerors appropriate the females and transmit their qualities to their offspring.

These are all cases of the survival of the fittest; of the survival of the males best adapted to the exigencies of their surroundings—in short, additional illustrations of Natural Selection.

Insect Selection. Flowers usually consist of several parts, such as the stem, the calyx composed of green sepals, the corolla formed of attractively colored petals, the stamens, the pistil, and finally of certain nectar-forming organs situated at the base of the last-named. The upper portions of the stamens are known as antheridia. The free extremity of each pistil is called a stigma, the intermediate stalk is the style, and the base is the ovary. The flowers constitute the sexual parts of the plant. The stamens are the male elements and the pistils are the female structures. In some species of plants the flowers possess the stamens but not the pistils,—they are male flowers; in others the flowers have pistils but not stamens,—they are female flowers. In still other plants the flowers possess both the male and the female structures, and are therefore bisexual or hermaphroditic flowers. The ovary of a pistil has a number of cells in it called ovules (female germ cells); the antheridia on the stamens have cells in them called pollen (male germ cells). When a pollen cell is carried by any means to the stigma of a pistil, it sends down through the latter a tubular prolongation by which the nucleus and protoplasm of the pollen cell unite with the nucleus and protoplasm of the ovule; so that we now have a fertilized egg,—the germ of a new plant. This fertilized ovule by repeated cell multiplication can grow into an adult plant.

There are two agencies by which the pollen is carried to the pistil; first, by the wind, and, secondly, by insects (or, occasionally, by humming birds). Flowers that are fertilized through the agency of insects are the most beautiful in existence, displaying all the varied hues and gorgeous patterns that are found in the organic world. On the other hand, the flowers which are fertilized through the agency of the wind are incomparably less beautiful than those of insect production, and they do not secrete sweet juices or nectar. The earliest flowers in geologic time consisted only of those essential portions, the stamens and pistils, and had no colored whorl of petals within another colored whorl of sepals. The poorly developed nectaries secreted only small quantities of honey. The food-seeking insects visited these primitive flowers for the pollen and nectar, even as they do now. The nectaries in the plants were so situated that the insects could not get at the honey without rubbing off some of the pollen and carrying it to other plants which it visited for honey. In this way the nectar-seeking insects would often carry pollen from the flower of one plant to the stigma of another plant’s flower, and thus secure cross-fertilization. Undoubtedly often the stigma of a plant’s flower was fertilized by pollen from the same flower; but it is known that seeds produced by cross-fertilization are much more vigorous and hardy than those produced by self-fertilization in the same flower; therefore, those plants that varied most in the direction best adapted for securing cross-fertilization would have decided advantages in the struggle for existence. Their fertilized eggs (seeds) would be most vigorous, and would most readily secure nutriment and withstand adverse circumstances. The variations of the primitive flowers most calculated to secure cross-fertilization would be those that made the flower more conspicuous by the appearance of bright colorations and delightful odors, such as would attract the attention and visits of nectar-seeking insects. These variations being transmitted, by heredity, to the descendants and accentuated as the ages passed would ultimately lead to all the wonderful adaptations of flower and insect structure to one another that are found in nature.

Plate IX.—Two Primrose flowers (Primula elatior). A, with a long style; B, with a short one. Vertical sections have been made through the flowers.

Natural Selection has acted in a double manner here, preserving those flowers that had the most delightful odors or the most conspicuous and enticing colorations and patterns; and at the same time selecting those insects that varied most in the direction of keenness of scent, acuteness of vision, and color perceptions. It will be instructive to briefly describe the fertilization of the primrose and an orchis by bees. The primrose ([Plate IX]) has two sorts of flowers that are never found on the same plant; one has stamens high up the corolla tube and a short style with its stigma below the anthers; the other has the stamens far down the tube and a long style with the stigma above the anthers. In both cases the nectar is at the very bottom of the corolla tube. When a bee visits a short-styled flower, it extends its proboscis to the bottom of the tube and in withdrawing it brings away some pollen cells at its base. If it should next visit another short-styled primrose, it would not fertilize it because the pollen on the base of the proboscis could not reach the stigma; it would only gather still more pollen. But when it visits a long-styled primrose, it will necessarily deposit some pollen cells on the stigma, inasmuch as that is at the commencement of the corolla tube. If the bee should first visit a long-styled form of flower, it will carry off pollen on the tip of its proboscis, and when it visits a short-styled primrose will deposit the pollen on the stigma.

In the orchis the stigma is placed below the anther. The stigma is in the front part of the flower and at the base of the lip, the latter being prolonged into a long tube full of nectar. The stigmatic surface is composed of very viscid matter. A bee when seeking the nectar pushes its head against the anther, and in so doing detaches the two sticky glands to which the club-shaped masses of pollen cells are attached ([Plate X]). It carries these away on the front part of its head. So long as the masses of pollen cells remain erect on the bee’s head, they cannot reach the stigma of any other orchis that it visits. As the sticky glands on the head contract, the pollen masses incline forward and become horizontal, so that they touch the next sticky stigma that is visited. The greater stickiness of the stigma detaches the pollen masses from the bee’s head, so that the flower is fertilized. It takes so long for the pollen masses on the bee’s head to assume the horizontal position that the insect has visited all the flowers on one orchis and then visits another plant. By this time the pollen masses are in the proper position (horizontal) to fertilize the flowers of another plant. In this way cross-fertilization is secured and the vigor of the plant maintained.

It may now perhaps be appreciated how intimate are the relations between the form and habits of insects and the structure and coloration of flowers. Our standards of beauty have largely been created for us through Insect and Sexual Selection. If insects had not been developed on the earth, plants would not be ornamented by beautiful flowers, but would show only such poor and inconspicuous ones as are found on grasses, on oak, ash, and fir trees. Grant Allen has beautifully written that, “while man has tilled only a few level plains, a few great river-valleys, a few peninsular mountain slopes, leaving the vast mass of earth untouched by his hand, the insect has spread itself over every land in a thousand shapes, and has made the whole flowering creation subservient to his daily wants. His buttercup, his dandelion, and his meadowsweet grow thick in every English field. His thyme clothes the hillside; his heather purples the bleak grey moorlands. High up among the Alpine heights his gentian spreads its lakes of blue; amid the snows of the Himalayas his rhododendrons gleam with crimson light. Even the wayside pond yields him the white crowfoot and the arrowhead, while the broad expanses of Brazilian streams are beautified by his gorgeous water lilies. The insect has turned the whole surface of the earth into a boundless flower-garden, which supplies him from year to year with pollen or honey, and itself in turn gains perpetuation by the baits that it offers to his allurement.”