This, then, was the earliest hypothesis touching the causes of organic evolution. But we may at once perceive that it is insufficient to explain all that stands to be explained. In the first place, it refers in chief part only to the higher animals, which are actuated to effort by intelligence. Its explanatory power in the case of most invertebrata--as well as in that of all plants--is extremely limited, inasmuch as these organisms can never be moved to a greater or less use of their several parts by any discriminating volition, such as that which leads to the continued straining of a giraffe's neck for the purpose of reaching foliage. In the second place, even among the higher animals there are numberless tissues and organs which unquestionably present a high degree of adaptive evolution, but which nevertheless cannot be supposed to have fallen within the influence of Lamarckian principles. Of such are the sh.e.l.ls of crustacea, tortoises, &c., which although undoubtedly of great use to the animals presenting them, cannot ever have been _used_ in the sense required by Lamarck's hypothesis, i.

e. actively exercised, so as to increase a flow of nutrition to the part. Lastly, in the third place, the validity of Lamarck's hypothesis in any case whatsoever has of late years become a matter of serious question, as will be fully shown and discussed in the next volume.

Meanwhile it is enough to observe that, on account of all these reasons, the theory of Lamarck, even if it be supposed to present any truth at all, is clearly insufficient as a full or complete theory of organic evolution.

In historical order the next theory that was arrived at was the theory of natural selection, simultaneously published by Darwin and Wallace on July 1st, 1858.

If we may estimate the importance of an idea by the change of thought which it effects, this idea of natural selection is unquestionably the most important idea that has ever been conceived by the mind of man.

Yet the wonder is that it should not have been hit upon long before. Or rather, I should say, the wonder is that its immense and immeasurable importance should not have been previously recognised. For, since the publication of this idea by Darwin and Wallace, it has been found that its main features had already occurred to at least two other minds--namely, Dr. Wells in 1813, and Mr. Patrick Matthew in 1831. But neither of these writers perceived that in the few scattered sentences which they had written upon the subject they had struck the key-note of organic nature, and resolved one of the princ.i.p.al chords of the universe. Still more remarkable is the fact that Mr. Herbert Spencer--notwithstanding his great powers of abstract thought and his great devotion of those powers to the theory of evolution, when as yet this theory was scorned by science--still more remarkable, I say, is the fact that Mr. Herbert Spencer should have missed what now appears so obvious an idea. But most remarkable of all is the fact that Dr.

Whewell, with all his stores of information on the history of the inductive sciences, and with all his ac.u.men on the matter of scientific method, should not only have conceived the idea of natural selection, but expressly stated it as a logically possible explanation of the origin of species, and yet have so stated it merely for the purpose of dismissing it with contempt[26]. This, I think, is most remarkable, because it serves to prove how very far men's minds at that time must have been from entertaining, as in any way antecedently probable, the doctrine of trans.m.u.tation. In order to show this I will here quote one pa.s.sage from the writings of Whewell, and another from a distinguished French naturalist referred to by him.

[26] For quotations, see Note A.

In 1846 Whewell wrote:--

Not only is the doctrine of the trans.m.u.tation of species in itself disproved by the best physiological reasonings, but the additional a.s.sumptions which are requisite to enable its advocates to apply it to the explanation of the geological and other phenomena of the earth, are altogether gratuitous and fantastical[27].

[27] whewell, _indications of the creator_, 2nd ed., 1846.

Then he quotes with approval the following opinion:--

Against this hypothesis, which, up to the present time, I regard as purely gratuitous, and likely to turn geologists out of the sound and excellent road in which they now are, I willingly raise my voice, with the most absolute conviction of being in the right[28].

[28] de blainville, _compte rendu_, 1837.

And, after displaying the proof rendered by Lyell of uniformitarianism in geology, and cordially subscribing thereto, Whewell adds:--

We are led by our reasonings to this view, that the present order of things was commenced by an act of creative power entirely different to any agency which has been exerted since. None of the influences which have modified the present races of animals and plants since they were placed in their habitations on the earth's surface can have had any efficacy in producing them at first. We are necessarily driven to a.s.sume, as the beginning of the present cycle of organic nature, an event not included in the course of nature[29].

[29] Whewell, _ibid._, p. 162.

So much, then, for the state of the most enlightened and representative opinions on the question of evolution before the publication of Darwin's work; and so much, likewise, for the only reasonable suggestions as to the causes of evolution which up to that time had been put forward, even by those few individuals who entertained any belief in evolution as a fact. It was the theory of natural selection that changed all this, and created a revolution in the thought of our time, the magnitude of which in many of its far-reaching consequences we are not even yet in a position to appreciate; but the action of which has already wrought a transformation in general philosophy, as well as in the more special science of biology, that is without a parallel in the history of mankind.

Although every one is now more or less well acquainted with the theory of natural selection, it is necessary, for the sake of completeness, that I should state the theory; and I will do so in full detail.

It is a matter of observable fact that all plants and animals are perpetually engaged in what Darwin calls a "struggle for existence."

That is to say, in every generation of every species a great many more individuals are born than can possibly survive; so that there is in consequence a perpetual battle for life going on among all the const.i.tuent individuals of any given generation. Now, in this struggle for existence, which individuals will be victorious and live? a.s.suredly those which are best fitted to live, in whatever respect, or respects, their superiority of fitness may consist. Hence it follows that Nature, so to speak, _selects_ the best individuals out of each generation to live. And not only so; but as these favoured individuals transmit their favourable qualities to their offspring, according to the fixed laws of heredity, it further follows that the individuals composing each successive generation have a general tendency to be better suited to their surroundings than were their forefathers. And this follows, not merely because in every generation it is only the "flower of the flock"

that is allowed to breed, but also because, if in any generation some new and beneficial qualities happen to arise as slight variations from the ancestral type, they will (other things permitting) be seized upon by natural selection, and, being transmitted by heredity to subsequent generations, will be added to the previously existing type. Thus the best idea of the whole process will be gained by comparing it with the closely a.n.a.logous process whereby gardeners, fanciers, and cattle-breeders create their wonderful productions; for just as these men, by always "_selecting_" their best individuals to breed from, slowly but continuously improve their stock, so Nature, by a similar process of "_selection_" slowly but continuously makes the various species of plants and animals better and better suited to the conditions of their life.

Now, if this process of continuously adapting organisms to their environment takes place in nature at all, there is no reason why we should set any limits on the extent to which it is able to go, up to the point at which a complete and perfect adaptation is achieved. Therefore we might suppose that all species would eventually reach this condition of perfect harmony with their environment, and then remain fixed. And so, according to the theory, they would, if the environment were itself unchanging. But forasmuch as the environment (i. e. the sum total of the external conditions of life) of almost every organic type alters more or less from century to century--whether from astronomical, geological, and geographical changes, or from the immigrations and emigrations of other species living on contiguous areas, and so on--it follows that the process of natural selection need never reach a terminal phase. And forasmuch as natural selection may thus continue, _ad infinitum_, slowly to alter a specific type in adaptation to a gradually changing environment, if in any case the alteration thus effected is sufficient in amount to lead naturalists to name the result as a distinct species, it follows that natural selection has trans.m.u.ted one specific type into another. Similarly, by a continuation of the process, specific types would become trans.m.u.ted into generic, generic into family types, and so on. Thus the process is supposed to go on throughout all the countless forms of life continuously and simultaneously--the world of organic types being thus regarded as in a state of perpetual, though gradual, flux.

Now, the first thing we have to notice about this theory is, that in all its main elements it is merely a statement of observable facts. It is an observable fact that in all species of plants and animals a very much larger number of individuals are born than can possibly survive. Thus, for example, it has been calculated that if the progeny of a single pair of elephants--which are the slowest breeding of animals--were all allowed to reach maturity and propagate, in 750 years there would be living 19,000,000 descendants. Again, in the case of vegetables, if a species of annual plant produces only two seeds a year, if these in successive years were all allowed to reproduce their kind, in twenty years there would be 11,000,000 plants from a single ancestor. Yet we know that nearly all animals and plants produce many more young at a time than in either of these two supposed cases. Indeed, as individuals of many kinds of plants, and not a few kinds of animals, produce every year several thousand young, we may make a rough estimate and say, that over organic nature as a whole probably not one in a thousand young are allowed to survive to the age of reproduction. How tremendous, therefore, must be the struggle for existence! It is thought a terrible thing in battle when one half the whole number of combatants perish. But what are we to think of a battle for life where only one in a thousand survives?

This, then, is the first fact. The second is the fact so long ago recognised, that the battle is to the strong, the race to the swift. The thousandth individual which does survive in the battle for existence--which does win the race for life--is, without question, one of the individuals best fitted to do so; that is to say, best fitted to the conditions of its existence considered as a whole. Nature is, therefore, always picking out, or selecting, such individuals to live and to breed.

The third fact is, that the individuals so selected transmit their favourable qualities to their offspring by heredity. There is no doubt about this fact, so far as we are concerned with it. For although, as I have already hinted, considerable doubt has of late years been cast upon Lamarck's doctrine of the hereditary transmission of _acquired_ characters, it remains as impossible as ever it was to question the hereditary transmission of what are called _congenital_ characters. And this is all that Darwin's theory necessarily requires.

The fourth fact is, that although heredity as a whole produces a wonderfully exact copy of the parent in the child, there is never a precise reduplication. Of all the millions of human beings upon the face of the earth, no one is so like another that we cannot see some difference; the resemblance is everywhere specific, nowhere individual.

Now this same remark applies to all specific types. The only reason why we notice individual differences in the case of the human type more than we do in the case of any other types, is because our attention is here more incessantly focussed upon these differences. We are compelled to notice them in the case of our own species, however small they may appear to a naturalist, because, unless we do so, we should not recognise the members of our own family, or be able to distinguish between a man whom we know is ready to do us an important service, and another man whom we know is ready to cut our throats. But our common mother Nature is able thus to distinguish between all her children. Her eyes are much more ready to detect small individual peculiarities than are the eyes of any naturalist. No slight variations in the cast of feature or disposition of parts, no minute difference in the arrangement of microscopical cells, can escape her ever vigilant attention. And, consequently, when among all the innumerable mult.i.tudes of individual variations any one arises which--no matter in how slight a degree--gives to that individual a better chance of success in the struggle for life, Nature chooses that individual to survive, and so to perpetuate the improvement in his or her progeny.

Now I say that all these several component parts of Darwinian doctrine are not matters of theory, but matters of fact. The only element of theory in his doctrine of evolution by natural selection has reference to the degree in which these observable facts, when thus brought together, are adequate to account for the process of evolution.

So much, then, as a statement of the theory of natural selection. But from this statement--i. e. from the theory of natural selection itself--there follow certain matters of general principle which it is important to bear in mind. These, therefore, I shall here proceed to mention.

First of all, it is evident that the theory is applicable as an explanation of organic changes in specific types only in so far as these changes are of _use_, or so far as such changes endow the species with better chances of success in the general struggle for existence. This is the only sense in which I shall always employ the terms use, utility, service, benefit, and so forth--that is to say, in the sense of life-preserving.

Next, it must be clearly understood that the life which it is the object, so to speak, of natural selection to preserve, is primarily the life of the _species_; not that of the _individual_. Natural selection preserves the life of the individual only in so far as this is conducive to that of the species. Wherever the life-interests of the individual clash with those of the species, that individual is sacrificed in favour of others who happen better to subserve the interests of the species.

For example, in all organisms a greater or less amount of vigour is wasted, so far as individual interests are concerned, in the formation and the nourishment of progeny. In the great majority of plants and animals an enormous amount of physiological energy is thus expended.

Look at the roe or the milt of a herring, for instance, and see what a huge drain has been made upon the individual for the sake of its species. Again, all unselfish instincts have been developed for the sake of the species, and usually against the interests of the individual. An ant which will allow her head to be slowly drawn from her body rather than relinquish her hold upon a pupa, is clearly acting in response to an instinct which has been developed for the benefit of the hive, though fatal to the individual. And, in a lesser degree, the parental instincts, wherever they occur, are more or less detrimental to the interests of the individual, though correspondingly essential to those of the race.

These ill.u.s.trations will serve to show that natural selection always works primarily for the life-interests of the species--and, indeed, only works for those of the individual at all in so far as the latter happen to coincide with the former. Or, otherwise stated, the object of natural selection is always that of producing and maintaining specific types in the highest degree of efficiency, no matter what may become of the const.i.tuent individuals. Which is a striking republication by Science of a general truth previously stated by Poetry:--

So careful of the type she seems, So careless of the single life.

Tennyson thus noted the fact, and a few years later Darwin supplied the explanation.

But of course in many, if not in the majority of cases, anything that adds to the life-sustaining power of the single life thereby ministers also to the life-sustaining power of the type; and thus we can understand why all mechanisms and instincts which minister to the single life have been developed--namely, because the life of the species is made up of the lives of all its const.i.tuent individuals. It is only where the interests of the one clash with those of the other that natural selection works against the individual. So long as the interests are coincident, it works in favour of both.

Natural selection, then, is a theory which seeks to explain by natural causes the occurrence of every kind of adaptation which is to be met with in organic nature, on the a.s.sumption that adaptations of every kind have primary reference to the preservation of species, and therefore also, as a general rule, to the preservation of their const.i.tuent individuals. And from this it follows that where it is for the benefit of a species to change its type, natural selection will effect that change, thus leading to a specific trans.m.u.tation, or the evolution of a new species. In such cases the old species may or may not become extinct. If the trans.m.u.tation affects the species as a whole, or throughout its entire range, of course _that_ particular type becomes extinct, although it does so by becoming changed into a still more suitable type in the course of successive generations. If, on the other hand, the trans.m.u.tation affects only a part of the original species, or not throughout its entire range, then the other parts of that species may survive for any number of ages as they originally were. In the one case there is a ladder-like trans.m.u.tation of species in time; in the other case a possibly tree-like multiplication of species in s.p.a.ce. But whether the evolution of species be thus serial in time or divergent in s.p.a.ce, the object of natural selection, so to speak, is in either case the same--namely, that of preserving all types which prove best suited to the conditions of their existence.

Once more, the term "struggle for existence" must be understood to comprehend, not only a compet.i.tion for life among contemporary individuals of the same species, but likewise a struggle by all such individuals taken collectively for the continuance of their own specific type. Thus, on the one hand, while there is a perpetual civil war being waged between members of the same species, on the other hand there is a foreign war being waged by the species as a whole against its world as a whole. Hence it follows that natural selection does not secure survival of the fittest as regards individuals only, but also survival of the fittest as regards types. This is a most important point to remember, because, as a general rule, these two different causes produce exactly opposite effects. Success in the civil war, where each is fighting against all, is determined by _individual_ fitness and _self-reliance_.

But success in the foreign war is determined by what may be termed _tribal_ fitness and _mutual dependence_. For example, among social insects the struggle for existence is quite as great between different tribes or communities, as it is between different individuals of the same community; and thus we can understand the extraordinary degree in which not only co-operative instincts, but also largely intelligent social habits, have here been developed[30]. Similarly, in the case of mankind, we can understand the still more extraordinary development of these things--culminating in the moral sense. I have heard a sermon, preached at one of the meetings of the British a.s.sociation, entirely devoted to arguing that the moral sense could not have been evolved by natural selection, seeing that the altruism which this sense involves is the very opposite of selfishness, which alone ought to have been the product of survival of the fittest in a struggle for life. And, of course, this argument would have been perfectly sound had Darwin limited the struggle for existence to individuals, without extending it to communities. But if the preacher had ever read Darwin's works he would have found that, when thus extended, the principle of natural selection is bound to work in favour of the co-operative instincts in the case of so highly social an animal as man; and that of these instincts conscience is the highest imaginable exhibition.

[30] For cases, see _Animal Intelligence_, in the chapters on Ants and Bees; and, for discussion of principles, _Mental Evolution in Animals_, in the chapters on Instinct.

What I have called tribal fitness--in contradistinction to individual fitness--begins with the family, developes in the community (herd, hive, clan, &c.), and usually ends with the limits of the species. On the one hand, however, it is but seldom that it extends so far as to embrace the entire species; while, on the other hand, it may in some cases, and as it were sporadically, extend beyond the species. In these latter cases members of different species mutually a.s.sist one another, whether in the way of what is called symbiosis, or in a variety of other ways which I need not wait to mention. For the only point which I now desire to make clear is, that all cases of mutual aid or co-operation, whether within or beyond the limits of species, are cases which fall under the explanatory sweep of the Darwinian theory[31].

[31] Prince Kropotkin in the _Nineteenth Century_ (Feb. 1888, Apr.

1891) has adduced a large and interesting body of facts, showing the great prevalence of the principle of co-operation in organic nature.

Another important point to notice is, that it const.i.tutes no part of the theory of natural selection to suppose that survival of the fittest must invariably lead to _improvement_ of type, in the sense of superior organization. On the contrary, if from change of habits or conditions of life an organic type ceases to have any use for previously useful organs, natural selection will not only allow these organs in successive generations to deteriorate--by no longer placing any selective premium upon their maintenance--but may even proceed to a.s.sist the agencies engaged in their destruction. For, being now useless, they may become even deleterious, by absorbing nutriment, causing weight, occupying s.p.a.ce, &c., without conferring any compensating benefit. Thus we can understand why it is that parasites, for example, present the phenomena of what is called _degeneration_, i. e. showing by their whole structure that they have descended from a possibly very much higher type of organization than that which they now exhibit. Having for innumerable generations ceased to require their legs, their eyes, and so forth, all such organs of high elaboration have either disappeared or become vestigial, leaving the parasite as a more or less effete representative of its ancestry.

These facts of degeneration, as we have previously seen, are of very general occurrence, and it is evident that their importance in the field of organic evolution as a whole has been very great. Moreover, it ought to be particularly observed that, as just indicated, the facts may be due either to a pa.s.sive _cessation_ of selection, or to an active _reversal_ of it. Or, more correctly, these facts are probably _always_ due to the cessation of selection, although in most cases where species in a state of nature are concerned, the process of degeneration has been both hastened and intensified by the super-added influence of the reversal of selection. In the next volume I shall have occasion to recur to this distinction, when it will be seen that it is one of no small importance to the general theory of descent.