A further series of remarkable examples of correlative adaptation is furnished by the different animals and plants which become degenerated through parasitic life or parasitism. No other change in the mode of life so much affects the shapes of organisms as the adoption of a parasitical life. Plants thereby lose their green leaves; as, for instance, our native parasitical plants, Orobanche, Lathræa, Monotropa. Animals which originally have lived freely and independently, but afterwards adopt a parasitical mode of life on other animals or plants, in the first place cease to use their organs of motion and their organs of sense. The loss of this activity is succeeded by the loss ofthe organs themselves, and thus we find, for example, many crabs, or crustacea, which in their youth possess a tolerably high degree of organization, viz., legs, antennæ, and eyes, in old age completely degenerate, living as parasites, without eyes, without apparatus of motion, and without antennæ. The lively, active form of youth, has become a shapeless, motionless lump. Only the most necessary organs of nutrition and propagation retain their activity; all the rest of the body has degenerated. Evidently these complete transformations are, to a large extent, the direct consequences of cumulative adaption, of the non-use and defective exercise of the organs, but a great portion of them must certainly be attributed also to correlative adaptation. (Compare Plate X. and XI.)
A seventh law of adaptation, the fourth in the group of direct adaptation, isthe law of divergent adaptation. By this law we indicate the fact that parts originally formed alike have developed in different ways under the influence of external conditions. This law of adaptation is extremely important for the explanation of the phenomenon of division of labour, or polymorphism. We can see this very easily in our own selves; for instance, in the activity of our two hands. We usually accustom our right hand to quite different work from that which we give our left, and in consequence of the different occupation there arises a different formation of the two hands. The right hand, which we use much more than the left, shows a stronger development of the nerves, muscles, and bones. The same applies to the whole arm. In most human beings the bones and flesh of the right arm are, in consequence of their being more employed, stronger and heavier thanthose of the left arm. Now, as the special use of the right arm has been adopted and transmitted by inheritance for thousands of years among Europeans, the stronger shape and size of the right arm have already become hereditary. P. Harting, an excellent Dutch naturalist, has shown by measuring and weighing newly-born children, that even in them the right arm is more developed than the left.
According to the same law of divergent adaptation, both eyes also frequently develop differently. If, for example, a naturalist accustoms himself always to use one eye for the microscope (it is better to use the left), then that eye will acquire a power different from that of the other, and this division of labour is of great advantage. The one eye will become more short-sighted, and better suited for seeing things near at hand; the other eye becomes, on the contrary, more long-sighted, more acute for looking at an object in the distance. If, on the other hand, the naturalist alternately uses both eyes for the microscope, he will not acquire the short-sightedness of the one eye and the compensatory degree of long-sight in the other, which is attained by a wise distribution of these different functions of sight between the two eyes. Here then again the function, that is the activity, of originally equally-formed organs can become divergent by habit; the function reacts again upon the form of the organ, and thus we find, after a long duration of such an influence, a change in the more delicate parts and the relative growth of the divergent organs, which in the end becomes apparent even in their coarser outlines.
Divergent adaptation can very easily be perceived among plants, especially in creepers. Branches of one and the same creeping plant, which originally were formed alike,acquire a completely different form and extent, a completely different degree of curvature and diameter of spiral winding, according as they twine themselves round a thinner or a thicker bar. The divergent change of form of parts originally identical in form, which tending in different directions develop themselves under different external conditions, can be distinctly demonstrated in many other examples. As this divergent adaptation interacts with progressive inheritance, it becomes the cause of a division of labour among the different organs.
An eighth and last law of adaptation we may callthe law of unlimited or infinite adaptation. By it we simply mean to express that we know of no limit to the variation of organic forms occasioned by the external conditions of existence. We can assert of no single part of an organism, that it is no longer variable, or that if it were subjected to new external conditions it would not be changed by them. It has never yet been proved by experience that there is a limit to variation. If, for example, an organ degenerates from non-use, this degeneration ends finally in a complete disappearance of the organ, as is the case with the eyes of many animals. On the other hand, we are able, by continual practice, habit, and the ever-increasing use of an organ, to bring it to a degree of perfection which we should at the beginning have considered to be impossible. If we compare the uncivilized savages with civilized nations, we find among the former a development of the organs of sense—sight, smell, and hearing—such as civilized nations can hardly conceive of. On the other hand, the brain, that is mental activity, among more civilized nations is developed to a degree of which the wild savages have no idea.
There appears indeed to be a limit given to the adaptability of every organism, by the “type” of its tribe or phylum; that is, by the essential fundamental qualities of this tribe, which have been inherited from a common ancestor, and transmitted by conservative inheritance to all its descendants. Thus, for example, no vertebrate animal can acquire the ventral nerve-chord of articulate animals, instead of the characteristic spinal marrow of the vertebrate animals. However, within this hereditary primary form, within this inalienable type, the degree of adaptability is unlimited. The elasticity and fluidity of the organic form manifests itself, within the type, freely in all directions, and to an unlimited extent. But there are some animals, as, for example, the parasitically degenerate crabs and worms, which seem to pass even the limit of type, and have forfeited all the essential characteristics of their tribe by an astonishing degree of degeneration. As to the adaptability of man, it is, as in all other animals, also unlimited, and since it is manifested in him above all other animals, in the modifications of the brain, there can be absolutely no limit to the knowledge which man in a further progress of mental cultivation may not be able to exceed. The human mind, according to the law of unlimited adaptation, enjoys an infinite perspective of becoming ever more and more perfect.
These remarks are sufficient to show the extent of the phenomena of Adaptation, and the great importance to be attached to them. The laws of Adaptation, or the facts of Variation caused by the influence of external conditions, are just as important as the laws of Inheritance. All phenomena of Adaptation, in the end, can be traced toconditions of nutrition of the organism, in the same way as the phenomena of Inheritance are referable to conditions of reproduction; but the latter, as well as the former, may further be traced to chemical and physical, that is to mechanical, causes. According to Darwin’s Theory of Selection the new forms of organisms, the transformations which artificial selection produces in the state of cultivation, and which natural selection produces in the state of nature, arise solely by the interaction of such causes.
Interaction of the Two Organic Formative Causes, Inheritance and Adaptation.—Natural and Artificial Selection.—Struggle for Existence, or Competition for the Necessaries of Life.—Disproportion between the Number of Possible or Potential, and the Number of Real or Actual Individuals.—Complicated Correlations of all Neighbouring Organisms.—Mode of Action in Natural Selection.—Homochromic Selection as the Cause of Sympathetic Colourings.—Sexual Selection as the Cause of the Secondary Sexual Characters.—Law of Separation or Division of Labour (Polymorphism, Differentiation, Divergence of Characters).—Transition of Varieties into Species.—Idea of Species.—Hybridism.—Law of Progress or Perfectioning (Progressus, Teleosis).
Interaction of the Two Organic Formative Causes, Inheritance and Adaptation.—Natural and Artificial Selection.—Struggle for Existence, or Competition for the Necessaries of Life.—Disproportion between the Number of Possible or Potential, and the Number of Real or Actual Individuals.—Complicated Correlations of all Neighbouring Organisms.—Mode of Action in Natural Selection.—Homochromic Selection as the Cause of Sympathetic Colourings.—Sexual Selection as the Cause of the Secondary Sexual Characters.—Law of Separation or Division of Labour (Polymorphism, Differentiation, Divergence of Characters).—Transition of Varieties into Species.—Idea of Species.—Hybridism.—Law of Progress or Perfectioning (Progressus, Teleosis).
Inorder to arrive at a right understanding of Darwinism, it is, above all, necessary that the two organic functions ofInheritance and Adaptation, which we spoke of in our last chapter, should be more closely examined. If we do not, on the one hand, examine the purely mechanical nature of these two physiological activities, and the various action of their different laws, and if, on the other hand, we do not consider how complicated the interaction of these different laws of Inheritance and Adaptation must be, we shall not be able to understand how these two functions, by themselves, have been able to produce all the variety ofanimal and vegetable forms, which, in fact, they have. We have, at least, hitherto been unable to discover any other formative causes besides these two, and if we rightly understand the necessary and infinitely complicated interaction of Inheritance and Adaptation, we do not require to look for other unknown causes for the change of organic forms. These two fundamental causes are, as far as we can see, completely sufficient.
Even long before Darwin had published his Theory of Selection, some naturalists, and especially Goethe, had assumed the interaction of two distinct formative tendencies—a conservative or preserving, and a progressive or changing formative tendency—as the causes of the variety of organic forms. The former was called by Goethe the centripetal or specifying tendency, the latter the centrifugal tendency, or the tendency to metamorphosis (p. 89). These two tendencies completely correspond with the two processes of Inheritance and Adaptation.Inheritanceis thecentripetalorinternal formative tendencywhich strives to keep the organic form in its species, to form the descendants like the parents, and always to produce identical things from generation to generation.Adaptation, on the other hand, which counteracts inheritance, is thecentrifugalorexternal formative tendency, which constantly strives to change the organic forms through the influence of the varying agencies of the outer world, to create new forms out of those existing, and entirely to destroy the constancy or permanency of species. Accordingly as Inheritance or Adaptation predominates in the struggle, the specific form either remains constant or changes into a new species. The degree of constancy of form in the different species of animals andplants, which obtains at any moment, is simply the necessary result of the momentary predominance which either of these two formative powers (or physiological activities) has acquired over the other.
If we now return to the consideration of the process of selection or choice, the outlines of which we have already examined, we shall be in a position to see clearly and distinctly that both artificial and natural selection rest solely upon the interaction of these two formative tendencies. If we carefully watch the proceedings of an artificial selector—a farmer or a gardener—we find that only these two constructive forces are used by him for the production of new forms. The whole art of artificial selection rests solely upon a thoughtful and wise application of the laws of Inheritance and Adaptation, and upon their being applied and regulated in an artistic and systematic manner. Here the will of man constitutes the selecting force.
The case of natural selection is quite similar, for it also employs merely these two organic constructive forces, these ingrained physiological properties of Adaptation and Heredity, in order to produce the different species. But the selecting principle or force, which inartificialselection is represented by the consciouswill of manacting for a definite purpose, consists innaturalselection of the unconsciousstruggle for existenceacting without a definite plan. What we mean by “struggle for existence” has already been explained in the seventh chapter. It is the recognition of this exceedingly important identity which constitutes one of the greatest of Darwin’s merits. But as this relation is very frequently imperfectly or falsely understood, it is necessary to examine it now more closely, and to illustrateby a few examples the operation of the struggle for life, and the operation of natural selectionby means ofthe struggle for life (Gen. Morph. ii. 231).
When considering the struggle for life, we started from the fact that the number of germs which all animals and plants produce is infinitely greater than the number of individuals which actually come to life and remain alive for a longer or shorter time. Most organisms produce during life thousands or millions of germs, from each of which, under favourable circumstances, a new individual might arise. In most animals and plants these germs are eggs, that is cells, which for their development require sexual fructification. But among the Protista, the lowest organisms, which are neither animals nor plants, and which propagate themselves only in a non-sexual manner, the germ-cells, or spores, require no fructification. Now, in all cases the number of unsexual, as well as of sexual germs, is out of all proportion to the number of actually living individuals of every species.
Taken as a whole, the number of living animals and plants on our earth remains always about the same. The number of places in the economy of nature is limited, and in most parts of the earth’s surface these places are always approximately occupied. Certainly there occur everywhere and in every year fluctuations in the absolute and in the relative number of individuals of all species. However, taken as a whole, these fluctuations are of little importance, and it is broadly the fact that the total number of all individuals remains, on an average, almost constant. There is a constant fluctuation, which depends on the fact that in one year or another one or other series of animals and plantspredominates, and that every year the struggle for life somewhat alters their relations.
Every single species of animals and plants would have densely peopled the whole earth’s surface in a short time, if it had not had to struggle against a number of enemies and hostile influences. Even Linnæus calculated that if an annual plant only produced two seeds (and there is not one which produces so few), it would have yielded in twenty years a million of individuals. Darwin has calculated of elephants, which of all animals seem the slowest to increase, that in seven hundred and fifty years the descendants of a single pair would amount to nineteen millions of individuals; this is supposing that every elephant, during its period of fertility (from the 30th to the 90th year), produced only three pairs of young ones, and survived itself to its hundredth year. In like manner the increase of the number of human beings—if calculated on the average proportion of births to population, and no hindrances to the natural increase stood in the way—would be such as to double the total in twenty-five years. In every century the total number of men would have increased sixteen-fold; whereas we know that the total number of human beings increases but slowly, and that the increase of population is very different in different countries. While European tribes spread over the whole globe, other tribes or species of men every year draw nearer to their complete extinction. This is the case especially with the redskins of America, and with the copper-coloured natives of Australia. Even if these races were to propagate more abundantly than the white Europeans, yet they would sooner or later succumb to the latter in the struggle for life. But of all humanindividuals, as of all other organisms, by far the majority perish at the earliest period of their lives. Of the immense quantity of germs which every species produce, only very few actually succeed in developing, and of these few it is again only a very small portion which attain to the age in which they can reproduce themselves (compare p.161).
From the disproportion between the immense excess of organic germs and the small number of chosen individuals which are actually able to continue in existence beside one another, there follows of necessity that universal struggle for life, that constant fight for existence, that perpetual competition for the necessaries of life, of which I gave a sketch in my seventh chapter. It is this struggle for life which brings natural selection into play, which in its turn is made use of by the interaction of the phenomena of Inheritance and Adaptation as a sifting agency, and which thus causes a continual change in all organic forms. In this struggle for acquiring the necessary conditions of existence, those individuals will always overpower their rivals who possess any individual privilege, any advantageous quality, of which their fellow competitors are destitute. It is true we are able only in the fewest cases (in those animals and plants best known to us) to form an approximate conception of the infinitely complicated interaction of the numerous circumstances, all of which here come into combination. Only think how infinitely varied and complicated are the relations of every single human being to the rest of mankind, and in general, to the whole of the surrounding outer world. But similar relations prevail also among all animals and plants which live together in one place. All influence one anotheractively or passively. Every animal and every plant struggles directly with a number of enemies, beasts of prey, parasitic animals, etc. Plants standing together struggle with one another for the space of ground requisite for their roots, for the necessary amount of light, air, moisture, etc. In like-manner, animals living together struggle with one another for their food, dwelling-place, etc. In this most active and complicated struggle, any personal superiority, however small, any individual advantage, may possibly decide the issue in favour of the one possessing it. This privileged individual remains the victor in the struggle, and propagates itself, while its fellow-competitors perish before they succeed in propagating themselves. The personal advantage which gave it the victory is transmitted by inheritance to its descendants, and by a further development may become so strongly marked as to cause us to consider the later generations as a new species.
The infinitely complicated correlations which exist between the organisms of every district, and which must be looked upon as the real conditions of the struggle for life, are mostly unknown to us, and are very difficult to discover. We have hitherto been able to trace them only to a certain point in individual cases, as in the example given by Darwin of the relations between cats and red clover in England. The red clover (Trifolium pratense), which in England is among the best fodder for cattle, requires the visit of humming-bees in order to attain the formation of seeds. These insects, while sucking the honey from the bottom of the flower, bring the pollen in contact with the stigma, and thus cause the fructification of the flower, which never takes place without it. Darwin hasshown by experiments, that red clover which is not visited by humming-bees does not yield a single seed. The number of bees is determined by the number of their enemies, the most destructive of which are the field-mice. The more the field-mice predominate, the less the clover is fructified. The number of field-mice, again, is dependent upon the number of their enemies, principally cats. Hence in the neighbourhood of villages and towns, where many cats are kept, there are plenty of bees. A great number of cats, therefore, is evidently of great advantage for the fructification of clover. This example may be followed still further, as has been done by Carl Vogt, if we consider that cattle which feed on red clover are one of the most important foundations of the wealth of England. Englishmen preserve their bodily and mental powers chiefly by making excellent meat—roast beef and beefsteak—their principal food. The English owe the superiority of their brains and minds over those of other nations in a great measure to their excellent meat. But this is clearly indirectly dependent upon the cats, which pursue the mice. We may, with Huxley, even trace the chain of causes to those old maids who cherish and keep cats, and, consequently, are of the greatest importance to the fructification of the clover and to the prosperity of England. From this example we can see that the further it is traced the wider is the circle of action and of correlation. We can with certainty maintain that there exist a great number of such correlations in every plant and in every animal, only we are not always able to point out and survey their concatenation as in the last instance.
Another remarkable example of important correlations is the following, given by Darwin. In Paraguay, there areno wild oxen and horses, as in the neighbouring parts of South America, both north and south of Paraguay. This surprising circumstance is explained simply by the fact that in that country a kind of small fly is very frequent, and is in the habit of laying its eggs in the navel of newly-born calves and foals. The newly-born animals die in consequence of this attack, and the small deadly fly is therefore the cause of oxen and horses never becoming wild in that district. Supposing that this fly were destroyed by some insect-eating bird, then these large mammals would grow wild in Paraguay, as well as in the neighbouring parts of South America; and as they would eat a quantity of certain species of plants, the whole flora, and, consequently again, the whole fauna of the country would become changed. It is hardly necessary to state, that at the same time the whole economy, and consequently the character, of the human population would alter.
Thus the prosperity, nay, even the existence of whole populations can be indirectly determined by a single small animal or vegetable form in itself extremely insignificant. There are small coral islands whose human inhabitants live almost entirely upon the fruit of a species of palm. The fructification of this palm is principally effected by insects, which carry the pollen from the male to the female palm trees. The existence of these useful insects is endangered by insect-eating birds, which in their turn are pursued by birds of prey. The birds of prey, however, often succumb to the attack of a small parasitical mite, which develops itself in millions in their feathers. This small, dangerous parasite, again, may be killed by parasitical moulds. Moulds, birds of prey, and insects would in this case favour the prosperityof the palm, and consequently of man; birds, mites, and insect-eating birds would, on the other hand, endanger it.
Interesting examples in relation to the change of correlations in the struggle for life are furnished also by those isolated oceanic islands, uninhabited by man, on which at different times goats and pigs have been placed by navigators. These animals become wild, and having no enemies, they increase in number so excessively, that the rest of the animal and vegetable population suffer in consequence, and the island finally may become almost a waste, because there is insufficient food for the large mammals which increase too numerously. In some cases on an island thus overrun with goats and pigs, other navigators have let loose a couple of dogs, who enjoyed this superabundance of food, and they again increased so numerously, and made such havoc among the herds, that after several years the dogs themselves lacked food, and they also almost died out. The equilibrium of species continually changes in this manner in nature’s economy, accordingly as one or another species increases at the expense of the rest. In most cases the relations of different species of animals and plants to one another are much too complicated for us to be able to follow them, and I leave it to the reader to picture to himself what an infinitely complicated machinery is at work in every part of the world in consequence of this struggle. The impulses which started the struggle, and which altered and modified it in different places, are in the end seen to be the impulses of self-preservation—in fact, the instinct leading individuals to preserve themselves (the instinct of obtaining food), and the instinct leading them to preserve the species (instinct of propagation). It is these two fundamental instincts oforganic self-preservation of which Schiller, the idealist (not Goethe, the realist!) says:
“Meanwhile, until philosophySustains the structure of the world,Her workings will be carried onBy hunger and by love.”4
“Meanwhile, until philosophySustains the structure of the world,Her workings will be carried onBy hunger and by love.”4
It is these two powerful fundamental instincts which, by their varying activity, produce such extraordinary differences in species through the struggle for life. They are the foundations of the phenomena of Inheritance and Adaptation. We have, in fact, traced all phenomena of Inheritance to propagation, all phenomena of Adaptation to nutrition, as the two wider classes of material phenomena to which they belong.
The struggle for life in natural selection acts with as much selective power as does the will of man in artificial selection. The latter, however, acts according to a plan and consciously, the former without a plan and unconsciously. This important difference between artificial and natural selection deserves especial consideration. For we learn by it to understand howarrangements serving a purpose can be produced by mechanical causes acting without an object, as well as by causes acting for an object. The products of natural selection are arranged even more for a purpose than the artificial products of man, and yet they owe their existence not to a creative power acting for a definite purpose, but to a mechanical relation acting unconsciouslyand without a plan. If we had not thoroughly considered the interaction of Inheritance and Adaptation under the influence of the struggle for life, we should not at first be inclined to expect such results from this natural process of selection as are, in fact, furnished by it. It may therefore be appropriate here to mention a few especially striking examples of the activity of natural selection.
Let us first takeDarwin’s homochromic selectionof animals, or the so-called “sympathetic selection of colours,” into consideration. Earlier naturalists have remarked that numerous animals are of nearly the same colour as their dwelling-place, or the surroundings in which they permanently live. Thus, for example, plant-lice and many other insects living on leaves are of a green colour. The inhabitants of the deserts, the jerboa, or leaping mice, foxes of the desert, gazelles, lions, etc., are mostly of a yellow or yellowish-brown colour, like the sand of the desert. The polar animals, which live on the ice and snow, are white or grey, like ice and snow. Many of these animals change their colour in summer and winter. In summer, when the snow partly vanishes, the fur of these polar creatures becomes brownish-grey or blackish, like the naked earth, while in winter it again becomes white. Butterflies and insects which hover round the gay and bright flowers are like them in colour. Now, Darwin explains this surprising circumstance quite simply by the fact that such colours as agree with the colour of the habitation are of the greatest use to the animals concerned. If these animals are animals of prey, they will be able to approach the object of their pursuit more safely and with less likelihood of observation, and, in like manner, those animals which are pursued willbe able to escape more easily, if their colour is as little different as possible from that of their surroundings. If therefore originally an animal species varied so as to present cases of all colours, those individuals whose colour most resembled the surroundings must have been most favoured in the struggle for life. They remained more unobserved, maintained and propagated themselves, while those individuals or varieties differently coloured died out.
I have tried to explain, by the same sympathetic selection of colour, the wonderful fact that the majority of pelagic animals—that is, of those which live on the surface of the open sea—are bluish, or completely colourless and transparent, like glass and water itself. Such colourless, glassy animals are met with in the most different classes. To them belong, among fish, the Helmicthyidæ, through whose crystalline bodies the words of a book can be read; among the molluscs, the finned snails (Heteropods) and sea-butterflies, or whales-food (Pteropods); among worms, the Salpæ, Alciope, and Sagitta; further, a great number of pelagic crabs (Crustacea), and the greater part of the Medusæ Umbrella-jellies, (Discomedusæ); Comb-jellies, (Ctenophora). All of these pelagic animals, which float on the surface of the ocean, are transparent and colourless, like glass and like the water itself, while their nearest kin live at the bottom of the ocean, and are coloured and opaque like the inhabitants of the land. This remarkable fact, like the sympathetic colouring of the inhabitants of the earth, can be explained by natural selection. Among the ancestors of the pelagic glass-like animals which showed a different degree of colourlessness and transparency, those that were the most colourless and transparent must have been most favouredin the active struggle for life which takes place on the surface of the ocean. They were enabled to approach their prey the most easily unobserved, and were themselves least observed by their enemies. Hence they could preserve and propagate themselves more easily than their more coloured and opaque relatives; and finally, by accumulative adaptation and transmission by inheritance, through natural selection, in the course of many generations their bodies would attain that degree of crystal-like transparency and colourlessness which we at present admire in them. (Gen. Morph. ii. 242.)
No less interesting and instructive than homochromic selection is that species of natural selection which Darwin calls “sexual selection,” which explains the origin of the so-called “secondary sexual characters.” We have already mentioned these subordinate sexual characteristics, so instructive in many respects. They comprise those peculiarities of animals and plants which belong only to one of the two sexes, and which do not stand in any direct relation to the act of propagation itself (compare above, p. 244). Such secondary sexual characters occur in great variety among animals. We all know how striking is the difference of the two sexes in size and colour in many birds and butterflies. The male sex is generally the larger and more beautiful. It often possesses special decorations or weapons; as for example, the spur and comb of the cock, the antlers of the stag and deer, etc. All these peculiarities of the two sexes have nothing directly to do with propagation itself, which is effected by the “primary sexual characters,” or actual sexual organs.
Now, the origin of these remarkable “secondary sexual characters” is explained by Darwin simply by a choice orselection which takes place in the propagation of animals. In most animals the number of individuals of both sexes is unequal; either the number of the female or the number of the male individuals is greater, and, as a rule, when the season of propagation approaches, a struggle takes place between the rivals for the possession of the animals of the other sex. It is well known with what vigour and vehemence this struggle is fought out among the higher animals—among mammals and birds—especially among those of polygamous habits. Among gallinaceous birds, where for one cock there are several hens, a severe struggle takes place between the competing cocks for as large a harem as possible. The same is the case with many ruminating animals. Among stags and deer, for instance, at the period of rut, deadly struggles take place between the males for the possession of the females. The secondary sexual character which here distinguishes the males—the antlers of stags and deer—not possessed by the female, is, according to Darwin, the consequence of that struggle. Here the motive and cause determining the struggle is not, as in the case of the struggle for individual existence, self-preservation, but the preservation of the species—propagation. There are numerous passive weapons of defence, as well as active weapons for attack. The lion’s mane, not possessed by the female, is evidently such a weapon of defence; it is an excellent means of protection against the bites which the male lions try to inflict on each other’s necks when fighting for the females; consequently those males with the strongest manes have the greatest advantage in the sexual struggle. The dewlap of the ox and the comb of the cock are similar defensive weapons. Active weapons of attack, on the otherhand, are the antlers of the stag, the tusks of the boar, the spur of the cock, and the hugely developed pair of jaws in the male stag-beetle; all are instruments employed by the males in the struggle for the females, for annihilating or chasing away their rivals.
In the cases just mentioned, it is the bodily “struggle to the death” which determines the origin of the secondary sexual characters. But, besides these mortal struggles, there are other important competitions in sexual selection, which no less influence the structure of the rivals. These consist principally in the fact that the courting sex tries to please the other by external finery, by beauty of form, or by a melodious voice. Darwin thinks that the beautiful voices of singing birds have principally originated in this way. Many male birds carry on a regular musical contest when they contend for the possession of the females. It is known of several singing birds, that in the breeding season the males assemble in numbers round the females, and let their songs resound before them, and that then the females choose the singers who best please them for their mates. Among other songsters, individual males pour out their songs in the loneliness of the forest in order to attract the females, and the latter follow the most attractive calls. A similar musical contest, though certainly less melodious, takes place among crickets and grasshoppers. The male cricket has on its belly two instruments like drums, and produces with these the sharp chirping notes which the ancient Greeks curiously enough thought beautiful music. Male grasshoppers, partly by using their hind-legs like the bow of a violin against their wing coverings, and partly by rubbing their wing coverings together, bring out tones which are, indeed, notmelodious to us, but which please the female grasshoppers so much that they choose the male who fiddles the best.
Among other insects and birds it is not song or, in fact, any musical accomplishment, but finery or beauty of the one sex which attracts the other. Thus we find that, among most gallinaceous birds, the cocks are distinguished by combs on their heads, or by a beautiful tail, which they can spread out like a fan; as for example, in the case of the peacock and turkey-cock. The magnificent tail of the bird of paradise is also an exclusive ornament of the male sex. In like manner, among very many other birds and very many insects, principally among butterflies, the males are distinguished from the females by special colours or other decorations. These are evidently the results of sexual selection. As the females do not possess these attractions and decorations, we must come to the conclusion that they have been acquired by degrees by the males in the competition for the females, which takes its origin in the selective discrimination of the females.
We may easily picture to ourselves, in detail, the application of this interesting conclusion to the human community. Here, also, the same causes have evidently influenced the development of the secondary sexual characters. The characteristics distinguishing the man, as well as those distinguishing the woman, owe their origin, certainly for the most part, to the sexual selection of the other sex. In antiquity and in the Middle Ages, especially in the romantic age of chivalry, it was the bodily struggles to the death—the tournaments and duels—which determined the choice of the bride; the strongest carried home the bride. In more recent times, however, in our so-called “polished” or “highly civilized”society, competing rivals prefer to contend indirectly by means of musical accomplishments, instrumental performances and song, by bodily charms, natural beauty, or artificial decoration. But by far the most important of these different forms of sexual selection in man is that form which is the most exalted, namely,psychical selection, in which the mental excellencies of the one sex influence and determine the choice of the other. The most highly intellectually developed types of men have, throughout generations, when choosing a partner in life, been guided by her excellencies of soul, and have thus transmitted these qualities to their posterity, and they have in this way, more than by any other thing, helped to create the deep chasm which at present separates civilized men from the rudest savages, and from our common animal ancestors. In fact, both the part played by the prevalence of a higher standard of sexual selection, and the part played by the due division of labour between the two sexes, is exceedingly important, and I believe that here we must seek for the most powerful causes which have determined the origin and the historical development of the races of man. (Gen. Morph. ii. 247.) As Darwin, in his exceedingly interesting work, published in 1871, on “The Origin of Man and Sexual Selection,”(48)has discussed this subject in the most masterly manner, and has illustrated it by most remarkable examples, I refer for further detail to that work.
But now let us look again at two extremely important organic laws which can be explained by the theory of selection, as necessary consequences of natural selection in the struggle for existence. I mean the law ofdivision of labour, ordifferentiation, and the law ofprogress, orperfecting. When the phenomena due to these two laws first became known, through observation of the historical development, the individual development, and the comparative anatomy of animals and plants, naturalists were inclined to trace them to a direct creative influence. It was supposed to be part of the plan of the Creator, acting for a definite purpose, in the course of time to develop the forms of animals and plants more and more variously, and to bring them more and more to a state of perfection. We shall evidently make a great advance in the knowledge of nature if we reject this teleological and anthropomorphic conception, and if we can prove the two laws of Division of Labour and Perfecting to be the necessary consequences of natural selection in the struggle for life.
The first great law which follows directly and of necessity from natural selection, is that ofseparation, ordifferentiation, which is frequently calleddivision of labour, orpolymorphism, and which Darwin speaks of asdivergence of character. (Gen. Morph. ii. 249.) We understand by it the general tendency of all organic individuals to develop themselves more and more diversely, and to deviate from the common primary type. The cause of this general inclination towards differentiation and the formation of heterogeneous forms from homogeneous beginnings is, according to Darwin, simply to be traced to the circumstance that the struggle for life between every two organisms rages all the more fiercely the nearer the relation in which they stand to one another, or the more nearly alike they are. This is an exceedingly important, and in reality an exceedingly simple relation, but it is usually not duly considered.
It must be obvious to every one, that in a field of a certain size, beside the corn-plants which have been sown, agreat number of weeds can exist, and, moreover, in places which could not have been occupied by corn-plants. The more dry and sterile places of the ground, in which no corn-plant would thrive, may still furnish sustenance to weeds of different kinds; and such species and individuals of weeds will more readily be able to exist in such conditions, in proportion as they are suited to adapt themselves to the different parts of the ground. It is the same with animals. It is evident that a much greater number of animal individuals can live together in one and the same limited district, if they are of various and different natures, than if they are all alike. There are trees (for example, the oak) on which a couple of hundred of different species of insects live together. Some feed on the fruits of the tree, others on the leaves, others again on the bark, the root, etc. It would be quite impossible for an equal number of individuals to live on this tree if all were of one species; if, for example, all fed on the bark, or only upon the leaves. Exactly the same is the case in human society. In one and the same small town, only a certain number of workmen can exist, even when they follow different occupations. The division of labour, which is of the greatest use to the whole community, as well as to the individual workman, is a direct consequence of the struggle for life, of natural selection; for this struggle can be sustained more easily the more the activities, and hence, also, the forms of the different individuals deviate from one another. The different function naturally produces its reaction in changing the form, and the physiological division of labour necessarily determines the morphological differentiation, that is, the “divergence of character.”(37)
Now, I beg the reader again to remember that all speciesof animals and plants are variable, and possess the capability of adapting themselves to different places or to local relations. The varieties or races of each species, according to the laws of adaptation, deviate all the more from the original primary species, the greater the difference of the new conditions to which they adapt themselves. If we imagine these varieties—which have proceeded from a common primary form—to be disposed in the shape of a branching, radiating bunch, then those varieties will be best able to exist side by side and propagate which are most distant from one another, which stand at the ends of the series, or at the opposite sides of the bunch. Those forms, on the other hand, occupying a middle position—presenting a state of transition—have the most difficult position in the struggle for life. The necessaries of life differ most in the two extremes, in the varieties most distant from one another, and consequently these will get into the least serious conflict with one another in the general struggle for life. But the intermediate forms, which have deviated less from the original primary form, require nearly the same necessaries of life as the original form, and therefore, in competing for them, they will have to struggle most with, and be most seriously threatened by, its members. Consequently, when numerous varieties of a species live side by side on the same spot of the earth, the extremes, or those forms deviating most from one another, can much more easily continue to exist beside one another than the intermediate forms which have to struggle with each of the different extremes. The intermediate forms will not be able to resist, for any length of time, the hostile influences which the extreme forms victoriously overcome. These alone maintain and propagatethemselves, and at length cease to be any longer connected with the original primary species through intermediate forms of transition. Thus arise “good species” out of varieties. Thus, then, the struggle for life necessarily favours the general divergence of organic forms, that is, the constant tendency of organisms to form new species. This fact does not rest upon any mystic quality, or upon an unknown formative tendency, but upon the interaction of Inheritance and Adaptation in the struggle for life. As the intermediate forms, that is, the individuals in a state of transition, of the varieties of every species die out and become extinct, the process of divergence constantly goes further, and from the extremes forms develop which we distinguish as new species.
Although all naturalists have been obliged to acknowledge the variability and mutability of all species of animals and plants, yet most of them have hitherto denied that the modification or transformation of the organic form surpasses the original limit of the characters of the species. Our opponents cling to the proposition—“However far a species may exhibit deviations from its usual form in a collection of varieties, yet the varieties of it are never so distinct from one another as two really good species.” This assertion, which Darwin’s opponents usually place at the head of their arguments, is utterly untenable and unfounded. This will become quite clear as soon as we critically compare the various attempts to define the idea of species. No naturalist can answer the question as to what is in reality a “genuine or good species” (“bona species”); yet every systematic naturalist uses this expression every day, and whole libraries have been written on the question as towhether this or that observed form is a species or a variety, whether it is a really good or a bad species. The most general answer to this question used to be the following: “To one species belong all those individuals which agree in all essential characteristics. Essential characteristics of species are those which remain permanent or constant, and never become modified or vary.” But as soon as a case occurred in which the characteristic—which had hitherto been considered essential—did become modified, then it was said, “This characteristic is not essential to the species, for essential characteristics never vary.” Those who argued thus evidently moved in a circle, and the naïveté with which this circular method of defining species is laid down in thousands of books as an unassailable truth, and is still constantly repeated, is truly astonishing.
All other attempts which have been made to arrive at a definite and logical determination of the idea of organic “species” have, like the last, been utterly futile, and led to no results. Considering the nature of the case, it cannot be otherwise. The idea of species is just as truly a relative one and not absolute, as is the idea of variety, genus, family, order, class, etc. I have proved this in detail in the criticism of the idea of species in my “General Morphology” (Gen. Morph. ii. 323-364). I will waste no more time on this unsatisfactory discussion, and now only add a few words about therelation of species to hybridism. Formerly it was regarded as a dogma, that two good species could never produce hybrids which could reproduce themselves as such. Those who thus dogmatized almost always appealed to the hybrids of a horse and donkey, the mule and the hinny, which, truly enough, are seldom able to reproducethemselves. But the truth is that such unfruitful hybrids are rare examples, and in the majority of cases hybrids of two totally different species are fruitful and able to reproduce themselves. They can almost always fruitfully mix with one or other of the parent species, and sometimes also among themselves; and in this way completely new forms can originate according to the laws of “mixed transmission by inheritance.”
Thus, in fact,hybridism is a source of the origin of new species, distinct from the source we have hitherto considered—natural selection. I have already spoken occasionally of thesehybrid species(species hybridæ), especially of the hare-rabbit (Lepus Darwinii), which has arisen from the crossing of a male hare and a female rabbit; the goat-sheep (Capra ovina), which has arisen from the pairing of a he-goat and ewe; also the different species of thistles (Cirsium), brambles (Rubus), etc. It is possible that many wild species have originated in this way, as even Linnæus assumed. At all events, these hybrid species, which can maintain and propagate themselves as well as pure species, prove that hybridism cannot serve in any way to give an absolute definition to the idea of species.
I have already mentioned (p. 47) that the many vain attempts to define the idea of species theoretically have nothing whatever to do with the practical distinction of species. The extensive practical application of the idea of species, as it is carried out in systematic zoology and botany, is very instructive as furnishing an example of human folly. Hitherto, by far the majority of zoologists and botanists, in distinguishing and describing the different forms of animals and plants, have endeavoured, above all things, to distinguishaccurately kindred forms as so many “good species.” However, it has been found scarcely possible, in any group, to make an accurate and consistent distinction of such “genuine or good species.” There are no two zoologists, no two botanists, who agree in all cases as to which of the nearly related forms of a genus are good species, and which are not. All authors have different views about them. In the genusHieracium, for example, one of the commonest genera of European plants, no less than 300 species have been distinguished in Germany alone. The botanist Fries, however, only admits 106, Koch only 52, as “good species,” and others accept scarcely 20. The differences in the species of brambles (Rubus) are equally great. Where one botanist makes more than a hundred species, a second admits only about one half of that number, a third only five or six, or even fewer species. The birds of Germany have long been very accurately known. Bechstein, in his careful “Natural History of German Birds,” has distinguished 367 species, L. Reichenbach 379, Meyer and Wolff 406, and Brehm, a clergyman learned in ornithology, distinguishes even more than 900 different species.
Thus we see that here, and, in fact, in every other domain of systematic zoology and botany, the most arbitrary proceedings prevail, and, from the nature of the case, must prevail. For it is quite impossible accurately to distinguish varieties and races from so-called “good species.”Varieties are commencing species.The variability or adaptability of species, under the influence of the struggle for life, necessitates the continual and progressive separation or differentiation of varieties, and the perpetual delimitation of new forms. Whenever these are maintained throughout a number ofgenerations by inheritance, whilst the intermediate forms die out, they form independent “new species.” The origin of new species by division of labour, or separation, divergence, or differentiation of varieties, is therefore anecessary consequence of natural selection.(37)
The same kind of interest attaches to a second great law which we deduce from natural selection, and which is, indeed, closely connected with the law of Divergence, but in no way identical with it; namely, the law ofProgress(progressus), orPerfecting(teleosis). (Gen. Morph. ii. 257.) This great and important law, like the law of differentiation, had long been empirically established by palæontological experience, before Darwin’s Theory of Selection gave us the key to the explanation of its cause. The most distinguished palæontologists have pointed out the law of progress as the most general result of their investigations of fossil organisms. This has been specially done by Bronn, whose investigations on the laws of construction(18)and the laws of the development(19)of organisms, although little heeded, are excellent, and deserve most careful consideration. The general results of the law of differentiation and the law of progress, at which Bronn arrived by a purely mechanical hypothesis, and by exceedingly accurate, laborious, and careful investigations, are brilliant confirmations of the truth of these two great laws which we deduce as necessary inferences from the theory of selection.
The law of progress or of perfecting establishes the exceedingly important fact, on the ground of palæontological experience, that in successive periods of this earth’s history, a continual increase in the perfection of organic formations has taken place. Since that inconceivablyremote period in which life on our planet began with the spontaneous generation of Monera, organisms of all groups, both collectively as well as individually, have continually become more perfectly and highly developed. The steadily increasing variety of living forms has always been accompanied by progress in organization. The lower the strata of the earth in which the remains of extinct animals and plants lie buried, that is, the older the strata are, the more simple and imperfect are the forms which they contain. This applies to organisms collectively, as well as to every single large or small group of them, setting aside, of course, those exceptions which are due to the process of degeneration, which we shall discuss hereafter.
As a confirmation of this law I shall mention only the most important of all animal groups, the tribe of vertebrate animals. The oldest fossil remains of vertebrate animals known to us belong to the lowest class, that of Fishes. Upon these there followed later more perfect Amphibious animals, then Reptiles, and lastly, at a much later period, the most highly organized classes of vertebrate animals, Birds and Mammals. Of the latter only the lowest and most imperfect forms, without placenta, appeared at first, such as are the pouched animals (Marsupials), and afterwards, at a much later period, the more perfect mammals, with placenta. Of these, also, at first only the lower kinds appeared, the higher forms later; and not until the late tertiary period did man gradually develop out of these last.
If we follow the historical development of the vegetable kingdom we shall find the same law operative there. Of plants there existed at first only the lowest and most imperfect classes, the Algæ or tangles. Later there followedthe group of Ferns or Filicinæ (ferns, pole-reeds, scale-plants, etc.). But as yet there existed no flowering plants, or Phanerogama. These originated later with the Gymnosperms (firs and cycads), whose whole structure stands far below that of the other flowering plants (Angiosperms), and forms the transition from the group of fern-like plants to the Angiosperms. These latter developed at a still later date, and among them there were at first only flowering plants without corolla (Monocotyledons and Monochlamyds); only later were there flowering plants with a corolla (Dichlamyds). Finally, again, among these the lower polypetalous plants preceded the higher gamopetalous plants. The whole series thus constitutes an irrefutable proof of the great law of progressive development.
Now, if we ask what is the cause of this fact, we again, just as in the case of differentiation, come back to natural selection in the struggle for life. If once more we consider the whole process of natural selection, how it operates through the complicated interaction of the different laws of Inheritance and Adaptation, we shall recognize not only divergence of character, but also the perfecting of structure to be the direct and necessary result of it. We can trace the same thing in the history of the human race. Here, too, it is natural and necessary that the progressive division of labour constantly furthers mankind, and urges every individual branch of human activity into new discoveries and improvements. This progress itself universally depends on differentiation, and is consequently, like it, a direct result of natural selection in the struggle for life.