Part 2

DARWIN, CHARLES

THE DESCENT OF MAN: THE ARGUMENT IN BRIEF

Since evolution is probable for all other animals, it is probable for man. The human form has so much in common with the forms of other animals that community of descent is strongly suggested. Man, like other creatures, is subject to the struggle for existence. Evidence shows that it is likely that man is descended from a tailed and hairy quadruped that dwelt in trees. Man's mental power has been the chief factor in his advance, especially in his development of language. Conscience is due to social instincts, love of approbation, memory, imagination and religious feeling. Sexual selection in its effects upon human advancement. 45

WALLACE, ALFRED R.

MIMICRY AND OTHER PROTECTIVE RESEMBLANCES AMONG ANIMALS

The colours of animals are useful for concealment from their prey, from the creatures upon which they prey. The lion is scarcely visible as he crouches on the sand or among desert rocks and stones. Larks, quails and many other birds are so tinted and mottled that their detection is difficult. The polar bear, living amid ice and snow, is white. Reptiles and fish are so coloured as to be almost invisible in the grass or gravel where they rest. Many beetles and other insects are so like the leaves or bark on which they feed that when motionless they cannot be discerned. Some butterflies resemble dead, dry or decaying leaves so closely as to elude discovery. Every individual better protected by colour than others, has a better chance for life, and of transmitting his hues. Harmless beetles and flies are so like wasps and bees as to be left alone. 71

HUXLEY, THOMAS H.

EVOLUTION OF THE HORSE

The hoof of the horse is simply a greatly enlarged and thickened nail: four of his five toes are reduced to mere vestiges. His teeth are built of substances of varying hardness: they wear away at different rates presenting uneven grinding surfaces. Probable descent of the horse, link by link, especially as traced in the fossils of North America. Evolution has taken a long time: how long the physicist and the astronomer must decide. 101

HOWARD, LELAND O.

FIGHTING PESTS WITH INSECT ALLIES

A scale insect threatened with ruin the orchards of California. Professor C. V. Riley decided that the pest was a native of Australia. Mr. A. Hoebele observes in Australia that the pest is kept down by ladybirds. These are accordingly sent to California where they destroy the scale insect and restore prosperity among the fruit-growers. Another pest, of olive trees, is devoured by an imported ladybird of another species. This plan extended to Portugal and Egypt with success. Grasshoppers killed by a fungus cultivated for the purpose. Introduction into the United States of the insect which fertilizes the Smyrna fig. 123

ILES, GEORGE

THE STRANGE STORY OF THE FLOWERS: A CHAPTER IN MODERN BOTANY

Dress is important, whether natural or artificial. Because they catch dust on their clothes, bees, moths and butterflies have brought about myriad espousals of flower with flower. Colours and scents of blossoms attract insects. A flower which in form, scent or hue varies gainfully is likely to survive while others perish. All the parts of a flower are leaves in disguise. Floral modes of repulsion and defence. Plants which devour insects, a habit gradually acquired. The mesquit tree tells of water. Plants believed to indicate mineral veins. Seeds as emigrants equipped with wings or hooks. Parasitic plants and their degradation. Tenants that pay a liberal rent. The gardener as a creator of new flowers. The modern sugar beet due to Mons. Vilmorin. 139

THE NATURALIST AS INTERPRETER AND SEER

THE ORIGIN OF SPECIES: THE ARGUMENT IN SUMMARY

CHARLES DARWIN

On the view that species are only strongly marked and permanent varieties, and that each species first existed as a variety, we can see why it is that no line of demarcation can be drawn between species, commonly supposed to have been produced by special acts of creation, and varieties which are acknowledged to have been produced by secondary laws. On this same view we can understand how it is that in a region where many species of a genus have been produced, and where they now flourish, these same species should present many varieties; for where the manufactory of species has been active, we might expect, as a general rule, to find it still in action; and this is the case if varieties be incipient species. Moreover, the species of the larger genera, which afford the greater number of varieties or incipient species, retain to a certain degree the character of varieties; for they differ from each other by a less amount of difference than do the species of smaller genera. The closely allied species also of a larger genera apparently have restricted ranges, and in their affinities they are clustered in little groups round other species--in both respects resembling varieties. These are strange relations on the view that each species was independently created, but are intelligible if each existed first as a variety.

As each species tends by its geometrical rate of reproduction to increase inordinately in number; and as the modified descendants of each species will be enabled to increase by as much as they become more diversified in habits and structure, so as to be able to seize on many and widely different places in the economy of nature, there will be a constant tendency in natural selection to preserve the most divergent offspring of any one species. Hence, during a long-continued course of modification, the slight differences of characteristic of varieties of the same species, tend to be augmented into the greater differences characteristic of the species of the same genus. New and improved varieties will inevitably supplant and exterminate the older, less improved, and intermediate varieties; and thus species are rendered to a large extent defined and distinct objects. Dominant species belonging to the larger groups within each class tend to give birth to new and dominant forms; so that each large group tends to become still larger, and at the same time more divergent in character. But as all groups cannot thus go on increasing in size, for the world would not hold them, the more dominant groups beat the less dominant. This tendency in the large groups to go on increasing in size and diverging in character, together with the inevitable contingency of much extinction, explains the arrangement of all the forms of life in groups subordinate to groups, all within a few great classes, which has prevailed throughout all time. This grand fact of the grouping of all organic beings under what is called the Natural System, is utterly inexplicable on the theory of creation.

As natural selection acts solely by accumulating slight, successive, favourable variations, it can produce no great or sudden modifications; it can act only by short and slow steps. Hence, the canon of "Nature makes no leaps," which every fresh addition to our knowledge tends to confirm, is on this theory intelligible. We can see why throughout nature the same general end is gained by an almost infinite diversity of means, for every peculiarity when once acquired is long inherited, and structures already modified in many different ways have to be adapted for the same general purpose. We can, in short, see why nature is prodigal in variety, though niggard in innovation. But why this should be a law of nature if each species has been independently created no man can explain.

Many other facts are, as it seems to me, explicable on this theory. How strange it is that a bird, under the form of a woodpecker, should prey on insects on the ground; that upland geese which rarely or never swim, would possess webbed feet; that a thrush-like bird should dive and feed on sub-aquatic insects; and that a petrel should have the habits and structure fitting it for the life of an auk! and so in endless other cases. But on the view of each species constantly trying to increase in number, with natural selection always ready to adapt the slowly varying descendants of each to any unoccupied or ill-occupied place in nature, these facts cease to be strange, or might even have been anticipated.

We can to a certain extent understand how it is that there is so much beauty throughout nature; for this may be largely attributed to the agency of selection. That beauty, according to our sense of it, is not universal, must be admitted by every one who will look at some venomous snakes, at some fishes, and at certain hideous bats with a distorted resemblance to the human face. Sexual selection has given the most brilliant colours, elegant patterns, and other ornaments to the males, and sometimes to both sexes of many birds, butterflies and other animals. With birds it has often rendered the voice of the male musical to the female, as well as to our ears. Flowers and fruit have been rendered conspicuous by brilliant colours in contrast with the green foliage, in order that the flowers may be easily seen, visited and fertilized by insects, and the seeds disseminated by birds. How it comes that certain colours, sounds and forms should give pleasure to man and the lower animals, that is, how the sense of beauty in its simplest form was first acquired, we do not know any more than how certain odours and flavours were first rendered agreeable.

As natural selection acts by competition, it adopts and improves the inhabitants of each country only in relation to their co-inhabitants; so that we need feel no surprise at the species of any one country, although on the ordinary view supposed to have been created and specially adapted for that country, being beaten and supplanted by the naturalized productions from another land. Nor ought we marvel if all the contrivances in nature be not, as far as we can judge, absolutely perfect, as in the case even of the human eye; or if some of them be abhorrent to our ideas of fitness. We need not marvel at the sting of the bee, when used against an enemy, causing the bee's own death; at drones being produced in such great numbers for one single act, and being then slaughtered by their sterile sisters; at the astonishing waste of pollen by our fir trees; at the instinctive hatred of the queen bee for her own fertile daughters; at ichneumonidæ feeding within the living bodies of caterpillars; or at other such cases. The wonder indeed, is, on the theory of natural selection, that more cases of the want of absolute perfection have not been detected.

The complex and little known laws governing production of varieties are the same, as far as we can judge, with the laws which have governed the production of distinct species. In both cases physical conditions seem to have produced some direct and definite effect, but how much we cannot say. Thus, when varieties enter any new station, they occasionally assume some of the characters proper to the species of that station. With both varieties and species, use and disuse seem to have produced a considerable effect; for it is impossible to resist this conclusion when we look, for instance, at the logger-headed duck, which has wings incapable of flight, in nearly the same condition as in the domestic duck; or when we look at the burrowing tucu-tucu, which is occasionally blind, and then at certain moles, which are habitually blind and have their eyes covered with skin; or when we look at the blind animals inhabiting the dark caves of America and Europe. With varieties and species, correlated variation seems to have played an important part, so that when one part has been modified other parts have been necessarily modified. With both varieties and species, reversions to long-lost characters occasionally occur. How inexplicable on the theory of creation is the occasional appearance of stripes on the shoulders and legs of the several species of the horse-genus and of their hybrids! How simply is this fact explained if we believe that these species are all descended from a striped progenitor, in the same manner as the several domestic breeds of the pigeon are descended from the blue and barred rock pigeon!

On the ordinary view of each species having been independently created, why should specific characters, or those by which the species of the same genus differ from each other, be more variable than generic characters in which they all agree? Why, for instance, should the colour of a flower be more likely to vary in any one species of genus, if the other species possess differently coloured flowers, than if all possessed the same coloured flowers? If species are only well-marked varieties, of which the characters have become in a high degree permanent, we can understand this fact; for they have already varied since they branched off from a common progenitor in certain characters, by which they have come to be specifically different from each other; therefore these same characters would be more likely again to vary than the generic characters which have been inherited without change for an immense period. It is inexplicable on the theory of creation why a part developed in a very unusual manner in one species alone of a genus, and therefore, as we may naturally infer, of great importance to that species, should be eminently liable to variation; but, on our view, this part has undergone, since the several species branched off from a common progenitor, an unusual amount of variability and modification, and therefore we might expect the part generally to be still variable. But a part may be developed in the most unusual manner, like the wing of a bat, and yet not be more variable than any other structure, if the part be common to many subordinate forms, that is, if it has been inherited for a very long period; for in this case it will have been rendered constant by long-continued natural selection.

Glancing at instincts, marvellous as some are, they offer no greater difficulty than do corporeal structures on the theory of the natural selection of successive, slight, but profitable modifications. We can thus understand why nature moves by graduated steps in endowing certain animals of the same class with their several instincts. I have attempted to show how much light the principle of gradation throws on the admirable architectural powers of the hive-bee. Habit no doubt often comes into play in modifying instincts; but it certainly is not indispensable, as we see in the case of neuter insects, which leave no progeny to inherit the effects of long-continued habit. On the view of all the species of the same genus having descended from a common parent, and having inherited much in common, we can understand how it is that allied species, when placed under widely different conditions of life, yet follow nearly the same instincts; why the thrushes of temperate and tropical South America, for instance, line their nests with mud like our British species. On the view of instincts having been slowly acquired through natural selection, we need not marvel at some instincts being not perfect and liable to mistakes, and at many instincts causing other animals to suffer.

If species be only well-marked and permanent varieties, we can see at once why their crossed offspring should follow the same complex laws in their degrees and kinds of resemblance to their parents--in being absorbed into each other by successive crosses, and in other such points--as do the crossed offspring of acknowledged varieties. This similarity would be a strange fact, if species had been independently created and varieties had been produced through secondary laws.