Part 12 (1/2)

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 sh.o.r.es 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 b.u.t.terflies, which then appear for the first time.

We may a.s.sume that they spread with great rapidity and abundance in the rich floral world of the mid-Jura.s.sic. More than 13,000 species of Lepidoptera are known to-day, and there are probably twice that number yet to be cla.s.sified by the entomologist. But so far the Tertiary deposits have yielded only the fragmentary remains of about twenty individual b.u.t.terflies.

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 b.u.t.terflies. 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 circ.u.mstances, act in the way we call ”instinctive.” The problem is to find how this mechanism and its remarkable actions were slowly evolved.

In view of the innumerable and infinitely varied forms of ”instinct”

in the insect world we must restrict ourselves to a single ill.u.s.tration--say, the social life of the ants and the bees. We are not without indications of the gradual development of this social life. In the case of the ant we find that the Tertiary specimens--and about a hundred species are found in Switzerland alone, whereas there are only fifty species in the whole of Europe to-day--all have wings and are, apparently, of the two s.e.xes, not neutral. This seems to indicate that even in the mid-Tertiary some millions of years after the first appearance of the ant, the social life which we admire in the ants today had not yet been developed. The Tertiary bees, on the other hand, are said to show some traces of the division of labour (and modification of structure) which make the bees so interesting; but in this case the living bees, rising from a solitary life through increasing stages of social co-operation, give us some idea of the gradual development of this remarkable citizens.h.i.+p.

It seems to me that the great selective agency which has brought about these, and many other remarkable activities of the insects (such as the storing of food with their eggs by wasps), was probably the occurrence of periods of cold, and especially the beginning of a winter season in the Cretaceous or Tertiary age. In the periods of luxuriant life (the Carboniferous, the Jura.s.sic, or the Oligocene), when insects swarmed and varied in every direction, some would vary in the direction of a more effective placing of the eggs; and the supervening period of cold and scarcity would favour them. When a regular winter season set in, this tendency would be enormously increased. It is a parallel case to the evolution of the birds and mammals from the reptiles. Those that varied most in the direction of care for the egg and the young would have the largest share in the next generation. When we further reflect that since the Tertiary the insect world has pa.s.sed through the drastic disturbance of the climate in the great Ice-Age, we seem to have an illuminating clue to one of the most remarkable features of higher insect life.

The origin of the colour marks' and patterns on so many of the higher insects, with which we may join the origin of the stick-insects, leaf-insects, etc., is a subject of lively controversy in science to-day. The protective value of the appearance of insects which look almost exactly like dried twigs or decaying leaves, and of an arrangement of the colours of the wings of b.u.t.terflies which makes them almost invisible when at rest, is so obvious that natural selection was confidently invoked to explain them. In other cases certain colours or marks seemed to have a value as ”warning colours,” advertising the nauseousness of their possessors to the bird, which had learned to recognise them; in other cases these colours and marks seemed to be borrowed by palatable species, whose unconscious ”mimicry” led to their survival; in other cases, again, the patterns and spots were regarded as ”recognition marks,” by which the male could find his mate.

Science is just now pa.s.sing through a phase of acute criticism--as the reader will have realised by this time--and many of the positions confidently adopted in the earlier constructive stage are challenged.

This applies to the protective colours, warning colours, mimicry, etc., of insects. Probably some of the affirmations of the older generation of evolutionists were too rigid and extensive; and probably the denials of the new generation are equally exaggerated. When all sound criticism has been met, there remains a vast amount of protective colouring, shaping, and marking in the insect world of which natural selection gives us the one plausible explanation. But the doctrine of natural selection does not mean that every feature of an animal shall have a certain utility.

It will destroy animals with injurious variations and favour animals with useful variations; but there may be a large amount of variation, especially in colour, to which it is quite indifferent. In this way much colour-marking may develop, either from ordinary embryonic variations or (as experiment on b.u.t.terflies shows) from the direct influence of surroundings which has no vital significance. In this way, too, small variations of no selective value may gradually increase until they chance to have a value to the animal. [*]

* For a strong statement of the new critical position see Dewar and Finn's ”Making of Species,” 1909, ch. vi.

The origin of the metamorphosis, or pupa-stage, of the higher insects, with all its wonderful protective devices, is so obscure and controverted that we must pa.s.s over it. Some authorities think that the sleep-stage has been evolved for the protection of the helpless transforming insect; some believe that it occurs because movement would be injurious to the insect in that stage; some say that the muscular system is actually dissolved in its connections; and some recent experts suggest that it is a reminiscence of the fact that the ancestors of the metamorphosing insects were addicted to internal parasitism in their youth. It is one of the problems of the future. At present we have no fossil pupa-remains (though we have one caterpillar) to guide us. We must leave these fascinating but difficult problems of insect life, and glance at the evolution of the birds.

To the student of nature whose interest is confined to one branch of science the record of life is a mysterious Succession of waves. A comprehensive view of nature, living and non-living, past and present, discovers scores of illuminating connections, and even sees at times the inevitable sequence of events. Thus if the rise of the Angiospermous vegetation on the ruins of the Mesozoic world is understood in the light of geological and climatic changes, and the consequent deploying of the insects, especially the suctorial insects, is a natural result, the simultaneous triumph of the birds is not unintelligible. The grains and fruits of the Angiosperms and the vast swarms of insects provided immense stores of food; the annihilation of the Pterosaurs left a whole stratum of the earth free for their occupation.

We saw that a primitive bird, with very striking reptilian features, was found in the Jura.s.sic rocks, suggesting very clearly the evolution of the bird from the reptile in the cold of the Permian or Tria.s.sic period.

In the Cretaceous we found the birds distributed in a number of genera, but of two leading types. The Ichthyornis type was a tern-like flying bird, with socketed teeth and biconcave vertebrae like the reptile, but otherwise fully evolved into a bird. Its line is believed to survive in the gannets, cormorants, pelicans, and frigate-birds of to-day. The less numerous Hesperornis group were large and powerful divers. Then there is a blank in the record, representing the Cretaceous upheaval, and it unfortunately conceals the first great ramification of the bird world.

When the light falls again on the Eocene period we find great numbers of our familiar types quite developed. Primitive types of gulls, herons, pelicans, quails, ibises, flamingoes, albatrosses, buzzards, hornbills, falcons, eagles, owls, plovers, and woodc.o.c.ks are found in the Eocene beds; the Oligocene beds add parrots, trogons, cranes, marabouts, secretary-birds, grouse, swallows, and woodp.e.c.k.e.rs. We cannot suppose that every type has been preserved, but we see that our bird-world was virtually created in the early part of the Tertiary Era.

With these more or less familiar types were large ostrich-like survivors of the older order. In the bed of the sea which covered the site of London in the Eocene are found the remains of a toothed bird (Odontopteryx), though the teeth are merely sharp outgrowths of the edge of the bill. Another bird of the same period and region (Gastornis) stood about ten feet high, and must have looked something like a wading ostrich. Other large waders, even more ostrich-like in structure, lived in North America; and in Patagonia the remains have been found of a ma.s.sive bird, about eight feet high, with a head larger than that of any living animal except the elephant, rhinoceros, and hippopotamus (Chamberlin).

The absence of early Eocene remains prevents us from tracing the lines of our vast and varied bird-kingdom to their Mesozoic beginnings.

And when we appeal to the zoologist to supply the missing links of relations.h.i.+p, by a comparison of the structures of living birds, we receive only uncertain and very general suggestions. [*] He tells us that the ostrich-group (especially the emus and ca.s.sowaries) are one of the most primitive stocks of the bird world, and that the ancient Dinornis group and the recently extinct moas seem to be offshoots of that stock.

The remaining many thousand species of Carinate birds (or flying birds with a keel [carina]-shaped breast-bone for the attachment of the flying muscles) are then gathered into two great branches, which are ”traceable to a common stock” (Pycraft), and branch in their turn along the later lines of development. One of these lines--the pelicans, cormorants, etc.--seems to be a continuation of the Ichthyornis type of the Cretaceous, with the Odontopteryx as an Eocene offshoot; the divers, penguins, grebes, and petrels represent another ancient stock, which may be related to the Hesperornis group of the Cretaceous. Dr. Chalmers Mitch.e.l.l thinks that the ”screamers” of South America are the nearest representatives of the common ancestor of the keel-breasted birds. But even to give the broader divisions of the 19,000 species of living birds would be of little interest to the general reader.