The Story of Evolution

by Joseph McCabe

As this fresh green carpet spread over the formerly naked plains, it began to be enriched with our coloured flowers. There were large flowers, we saw, on some of the Mesozoic cycads, but their sober yellows and greens—to judge from their descendants—would do little to brighten the landscape. It is in the course of the Tertiary Era that the mantle of green begins to be embroidered with the brilliant hues of our flowers.

Grant Allen put forward in 1882 ("The Colours of Flowers") an interesting theory of the appearance of the colours of flowers, and it is regarded as probable. He observed that most of the simplest flowers are yellow; the more advanced flowers of simple families, and the simpler flowers of slightly advanced families, are generally white or pink; the most advanced flowers of all families, and almost all the flowers of the more advanced families, are red, purple, or blue; and the most advanced flowers of the most advanced families are always either blue or variegated. Professor Henslow adds a number of equally significant facts with the same tendency, so that we have strong reason to conceive the floral world as passing through successive phases of colour in the Tertiary Era. At first it would be a world of yellows and greens, like that of the Mesozoic vegetation, but brighter. In time splashes of red and white would lie on the face of the landscape; and later would come the purples, the rich blues, and the variegated colours of the more advanced flowers.

Why the colours came at all is a question closely connected with the general story of the evolution of the flower, at which we must glance. The essential characteristic of the flower, in the botanist's judgment, is the central green organ which you find—say, in a lily—standing out in the middle of the floral structure, with a number of yellow-coated rods round it. The yellow rods bear the male germinal elements (pollen); the central pistil encloses the ovules, or female elements. "Angiosperm" means "covered-seed plant," and its characteristic is this protection of the ovules within a special chamber, to which the pollen alone may penetrate. Round these essential organs are the coloured petals of the corolla (the chief part of the flower to the unscientific mind) and the sepals, often also coloured, of the calyx.

There is no doubt that all these parts arose from modifications of the leaves or stems of the primitive plant; though whether the bright leaves of the corolla are directly derived from ordinary leaves, or are enlarged and flattened stamens, has been disputed. And to the question why these bright petals, whose colour and variety of form lend such charm to the world of flowers, have been developed at all, most botanists will give a prompt and very interesting reply. As both male and female elements are usually in one flower, it may fertilise itself, the pollen falling directly on the pistil. But fertilisation is more sure and effective if the pollen comes from a different individual—if there is "cross fertilisation." This may be accomplished by the simple agency of the wind blowing the pollen broadcast, but it is done much better by insects, which brush against the stamens, and carry grains of the pollen to the next flower they visit.

We have here a very fertile line of development among the primitive flowers. The insects begin to visit them, for their pollen or juices, and cross-fertilise them. If this is an advantage, attractiveness to insects will become so important a feature that natural selection will develop it more and more. In plain English, what is meant is that those flowers which are more attractive to insects will be the most surely fertilised and breed most, and the prolonged application of this principle during hundreds of thousands of years will issue in the immense variety of our flowers. They will be enriched with little stores of honey and nectar; not so mysterious an advantage, when we reflect on the concentration of the juices in the neighbourhood of the seed. Then they must "advertise" their stores, and the strong perfumes and bright colours begin to develop, and ensure posterity to their possessors. The shape of the corolla will be altered in hundreds of ways, to accommodate and attract the useful visitor and shut out the mere robber. These utilities, together with the various modifying agencies of different environments, are generally believed to have led to the bewildering variety and great beauty of our floral world.

It is proper to add that this view has been sharply challenged by a number of recent writers. It is questioned if colours and scents do attract insects; though several recent series of experiments seem to show that bees are certainly attracted by colours. It is questioned if cross-fertilisation has really the importance ascribed to it since the days of Darwin. Some of these writers believe that the colours and the peculiar shape which the petals take in some flowers (orchises, for instance) have been evolved to deter browsing animals from eating them. The theory is thus only a different application of natural selection; Professor Henslow, on the other hand, stands alone in denying the selection, and believing that the insects directly developed the scents, honeys, colours, and shapes by mechanical irritation. The great majority of botanists adhere to the older view, and see in the wonderful Tertiary expansion of the flowers a manifold adaptation to the insect friends and insect foes which then became very abundant and varied.

Resisting the temptation to glance at the marvellous adaptations which we find to-day in our plant world—the insect-eating plants, the climbers, the parasites, the sensitive plants, the water-storing plants in dry regions, and so on—we must turn to the consideration of the insects themselves. We have already studied the evolution of the insect in general, and seen its earlier forms. The Tertiary Era not only witnessed a great deployment of the insects, but was singularly rich in means of preserving them. The "fly in amber" has ceased to be a puzzle even to the inexpert. Amber is the resin that exuded from pine-like trees, especially in the Baltic region, in the Eocene and Oligocene periods. Insects stuck in the resin, and were buried under fresh layers of it, and we find them embalmed in it as we pick up the resin on the shores of the Baltic to-day. The Tertiary lakes were also important cemeteries of insects. A great bed at Florissart, in Colorado, is described by one of the American experts who examined it as "a Tertiary Pompeii." It has yielded specimens of about a thousand species of Tertiary insects. Near the large ancient lake, of which it marks the site, was a volcano, and the fine ash yielded from the cone seems to have buried myriads of insects in the water. At Oeningen a similar lake-deposit has, although only a few feet thick, yielded 900 species of insects.

Yet these rich and numerous finds throw little light on the evolution of the insect, except in the general sense that they show species and even genera quite different from those of to-day. No new families of insects have appeared since the Eocene, and the ancient types had by that time disappeared. Since the Eocene, however, the species have been almost entirely changed, so that the insect record, from its commencement in the Primary Era, has the stamp of evolution on every page of it. Unfortunately, insects, especially the higher and later insects, are such frail structures that they are only preserved in very rare conditions. The most important event of the insect-world in the Tertiary is the arrival of the butterflies, which then appear for the first time. We may assume that they spread with great rapidity and abundance in the rich floral world of the mid-Jurassic. More than 13,000 species of Lepidoptera are known to-day, and there are probably twice that number yet to be classified by the entomologist. But so far the Tertiary deposits have yielded only the fragmentary remains of about twenty individual butterflies.

The evolutionary study of the insects is, therefore, not so much concerned with the various modifications of the three pairs of jaws, inherited from the primitive Tracheate, and the wings, which have given us our vast variety of species. It is directed rather to the more interesting questions of what are called the "instincts" of the insects, the remarkable metamorphosis by which the young of the higher orders attain the adult form, and the extraordinary colouring and marking of bees, wasps, and butterflies. Even these questions, however, are so large that only a few words can be said here on the tendencies of recent research.

In regard to the psychic powers of insects it may be said, in the first place, that it is seriously disputed among the modern authorities whether even the highest insects (the ant, bee, and wasp) have any degree whatever of the intelligence which an earlier generation generously bestowed on them. Wasmann and Bethe, two of the leading authorities on ants, take the negative view; Forel claims that they show occasional traces of intelligence. It is at all events clear that the enormous majority of, if not all, their activities—and especially those activities of the ant and the bee which chiefly impress the imagination—are not intelligent, but instinctive actions. And the second point to be noted is that the word "instinct," in the old sense of some innate power or faculty directing the life of an animal, has been struck out of the modern scientific dictionary. The ant or bee inherits a certain mechanism of nerves and muscles which will, in certain circumstances, act in the way we call "instinctive." The problem is to find how this mechanism and its remarkable actions were slowly evolved.