Victor Carus, of Leipzig, in the Introduction to his excellent "System of Animal Morphology,"(9) published in 1853, in which he endeavours to establish in a philosophical manner the universal constructive laws of the animal body through comparative anatomy and the history of development, makes the following remark:-"The organisms buried in the most ancient geological strata must be looked upon as the ancestors from whom the rich diversity of forms of the present creation have originated by continued generation, and by accommodation to progressive and very different conditions of life."

In the same year (1853) Schaaffhausen, the anthropologist of Bonn, in an Essay "On the Permanence and Transformation of Species," declared himself decidedly in favour of the Theory of Descent. According to him, the living species of animals and plants are the transformed descendants of extinct species, from which they have arisen by gradual modification.

The divergence or separation of the most nearly allied species takes place by the destruction of the connecting intermediate stages.

Schaaffhausen also maintained, with distinctness, the origin of the human race from animals, and its gradual development from ape-like animals, the most important deduction from the Doctrine of Filiation.

Lastly, we have still to mention among the German Nature-philosophers the name of Louis Buchner, who, in his celebrated work, "Force and Matter" (1855), also independently developed the principles of the Theory of Descent, taking his stand mainly on the ground of the undeniable evidences of fact which are furnished by the palaeontological and individual development of organisms, as well as by their comparative anatomy and by the parallelism of these series of development. Buchner showed very clearly that, even from such data alone, the derivation of the different organic species from common primary forms followed as a necessary conclusion, and that the origin of these original primary forms could only be conceived of as the result of a spontaneous generation.

We now turn from the German to the French Nature-philosophers, who have likewise held the Theory of Descent, since the beginning of the present century. At their head stands Jean Lamarck, who occupies the first place next to Darwin and Goethe in the history of the Doctrine of Filiation.

To him will always belong the immortal glory of having for the first time worked out the Theory of Descent, as an independent scientific theory of the first order, and as the philosophical foundation of the whole science of Biology. Although Lamarck was born as early as 1744, he did not begin the publication of his theory until the commencement of the present century, in 1801, and established it more fully only in 1809, in his cla.s.sic "Philosophie Zoologique."(2) This admirable work is the first connected exposition of the Theory of Descent carried out strictly into all its consequences. By its purely mechanical method of viewing organic nature, and the strictly philosophical proofs brought forward in it, Lamarck's work is raised far above the prevailing dualistic views of his time; and with the exception of Darwin's work, which appeared just half a century later, we know of none which we could in this respect place by the side of the "Philosophie Zoologique." How far it was in advance of its time is perhaps best seen from the circ.u.mstance that it was not understood by most men, and for fifty years was not spoken of at all. Cuvier, Lamarck's greatest opponent, in his "Report on the Progress of Natural Sciences," in which the most unimportant anatomical investigations are enumerated, does not devote a single word to this work, which forms an epoch in science. Goethe, also, who took such a lively interest in the French nature-philosophy and in "the thoughts of kindred minds beyond the Rhine," nowhere mentions Lamarck, and does not seem to have known the "Philosophie Zoologique" at all. The great reputation which Lamarck gained as a naturalist he does not owe to his highly important general work, but to numerous special treatises on the lower animals, particularly on Molluscs, as well as to an excellent "Natural History of Invertebrate Animals," which appeared, in seven volumes, between the years 1815-1822. The first volume of this celebrated work contains in the general introduction a detailed exposition of his theory of filiation. I can, perhaps, give no better idea of the extraordinary importance of the "Philosophie Zoologique"

than by quoting _verbatim_ some of the most important pa.s.sages therefrom:-

"The systematic divisions of cla.s.ses, orders, families, genera, and species, as well as their designations, are the arbitrary and artificial productions of man. The kinds or species of organisms are of unequal age, developed one after the other, and show only a relative and temporary persistence; species arise out of varieties. The differences in the conditions of life have a modifying influence on the organization, the general form, and the parts of animals, and so has the use or disuse of organs. In the first beginning only the very simplest and lowest animals and plants came into existence; those of a more complex organization only at a later period. The course of the earth's development, and that of its organic inhabitants, was continuous, not interrupted by violent revolutions. Life is purely a physical phenomenon. All the phenomena of life depend on mechanical, physical, and chemical causes, which are inherent in the nature of matter itself.

The simplest animals and the simplest plants, which stand at the lowest point in the scale of organization, have originated and still originate by spontaneous generation. All animate natural bodies or organisms are subject to the same laws as inanimate natural bodies or anorgana. The ideas and actions of the understanding are the motional phenomena of the central nervous system. The will is in truth never free. Reason is only a higher degree of development and combination of judgments."

These are indeed astonishingly bold, grand, and far-reaching views, and were expressed by Lamarck sixty years ago; in fact, at a time when their establishment, by a ma.s.s of facts, was not nearly as possible as it is in our day. Indeed Lamarck's work is really a complete and strictly monistic (mechanical) system of nature, and all the important general principles of monistic Biology are already enunciated by him: the unity of the active causes in organic and inorganic nature; the ultimate explanation of these causes in the chemical and physical properties of matter itself; the absence of a special vital power, or of an organic final cause; the derivation of all organisms from some few, most simple original forms, which have come into existence by spontaneous generation out of inorganic matter; the coherent course of the whole earth's history; the absence of violent cataclysmic revolutions; and in general the inconceivableness of any miracle, of any supernatural interference, in the natural course of the development of matter.

The fact that Lamarck's wonderful intellectual feat met with scarcely any recognition, arises partly from the immense length of the gigantic stride with which he had advanced beyond the next fifty years, partly from its defective empirical foundation, and from the somewhat one-sided character of some of his arguments. Lamarck quite correctly recognizes _Adaptation_ as the first mechanical cause which effects the continual transformation of organic forms, while he traces with equal justice the similarity in form of different species, genera, families, etc., to their blood-relationship, and thus explains it by _Inheritance_.

Adaptation, according to him, consists in this, that the perpetual, slow change of the outer world causes a corresponding change in the actions of organisms, and thereby also causes a further change in their forms.

He lays the greatest stress upon the effect of _habit_ upon the use and disuse of organs. This is certainly of great importance in the transformation of organic forms, as we shall see later. However, the way in which Lamarck wished to explain exclusively, or at any rate mainly, the change of forms, is after all in most cases not possible. He says, for example, that the long neck of the giraffe has arisen from its constantly stretching out its neck at high trees, and from the endeavour to pick the leaves off their branches; as giraffes generally inhabit dry districts, where only the foliage of trees afford them nourishment, they were forced to this action. In like manner the long tongues of wood-p.e.c.k.e.rs, humming-birds, and ant-eaters, are said by him to have arisen from the habit of fetching their food out of narrow, small, and deep crevices or channels. The webs between the toes of the webbed feet in frogs and other aquatic animals have arisen solely from the constant endeavour to swim, from striking their feet against the water, and from the very movements of swimming. Inheritance fixed these habits on the descendants, and finally, by further elaboration, the organs were entirely transformed. However correct, as a whole, this fundamental thought may be, yet Lamarck lays the stress too exclusively on _habit_ (use and non-use of organs), certainly one of the most important, but not the only cause of the change of forms. Still this cannot prevent our acknowledging that Lamarck quite correctly appreciated the mutual co-operation of the two organic formative tendencies of Adaptation and Inheritance. What he failed to grasp is the exceedingly important principle of "Natural Selection in the Struggle for Existence," with which Darwin, fifty years later, made us acquainted.

It still remains to be mentioned as a special merit of Lamarck, that he endeavoured to prove the _development of the human race_ from other primitive, ape-like mammals. Here again it was, above all, to habit that he ascribed the transforming, the enn.o.bling influence. He a.s.sumed that the lowest, original men had originated out of men-like apes, by the latter accustoming themselves to walk upright. The raising of the body, the constant effort to keep upright, in the first place led to a transformation of the limbs, to a stronger differentiation or separation of the fore and hinder extremities, which is justly considered one of the most essential distinctions between man and the ape. Behind, the calf of the leg and the flat soles of the feet were developed; in front, the arms and hands, for the purpose of seizing objects. The upright walk was then followed by a freer view over the surrounding objects, and led consequently to an important progress in mental development. Human apes thereby soon gained a great advantage over the other apes, and further, over surrounding organisms in general. In order to maintain the supremacy over them, they formed themselves into companies, and there arose, as in the case of all animals living in company, the desire of communicating to one another their desires and thoughts. Thus arose the necessity of language, which, consisting at first of rough and disjointed sounds, soon became more connected, developed, and articulate. The development of articulate speech now in turn became the strongest lever for a further progressive development of the organism, and above all, of the brain, and so ape-like men became gradually and slowly transformed into real men. In this way the actual descent of the lowest and rudest primitive men from the most highly developed apes was distinctly maintained by Lamarck, and supported by a series of the most important proofs.

The honour of being the chief French nature-philosopher is not usually a.s.signed to Lamarck, but to Etienne Geoffroy St. Hilaire (the elder), born in 1771, the same in whom Goethe was especially interested, and with whom we have already become acquainted as Cuvier's most prominent opponent. He developed his ideas about the transformation of organic species as far back as the end of the last century, but published them only in the year 1828, and then in the following years, especially in 1830, defended them bravely against Cuvier. Geoffroy St. Hilaire in all essentials adopted Lamarck's Theory of Descent, yet he believed that the transformation of animal and vegetable species was less effected by the action of the organism itself (by habit, practice, use, or disuse of organs) than by the "monde ambiant," that is, by the continual change of the outer world, especially of the atmosphere. He conceives the organism as pa.s.sive, in regard to the vital conditions of the outer world, while Lamarck, on the contrary, regards it as active. Geoffroy thinks, for example, that birds originated from lizard-like reptiles, simply by a diminution of the carbonic acid in the atmosphere, in consequence of which the breathing process became more animated and energetic through the increased proportion of oxygen in the atmosphere. Thus there arose a higher temperature of the blood, an increased activity of the nerves and muscles, and the scales of the reptiles became the feathers of the birds, etc. This conception is based upon a correct thought, but although the change of the atmosphere, as well as the change of every other external condition of existence, certainly effects directly or indirectly the transformation of the organism, yet this single cause is by itself too unimportant for such effects to be ascribed to it. It is even less important than practice and habit, upon which Lamarck lays too much stress. Geoffroy's chief merit consists in his having vindicated the monistic conception of nature, the unity of organic forms, and the deep genealogical connection of the different organic types in the face of Cuvier's powerful influence. I have already mentioned in the preceding chapter (pp. 87, 88) the celebrated disputes between the two great opponents in the Academy of Paris, especially the fierce conflicts on the 22nd of February, and on the 19th of July, in which Goethe took so lively an interest. On that occasion Cuvier remained the acknowledged victor, and since that time very little, or rather nothing, more has been done in France to further the development of the Doctrine of Filiation, and complete the monistic theory of development. This is evidently to be ascribed princ.i.p.ally to the repressive influence exercised by Cuvier's great authority. Even at the present day the majority of the French naturalists are the disciples and blind followers of Cuvier. In no civilized country of Europe has Darwin's doctrine had so little effect and been so little understood as in France, so that in the further course of our examination we need not take the French naturalists into consideration. At most, there are two distinguished botanists, among the recent French naturalists, whom we may mention as having ventured to express themselves in favour of the mutability and transformation of species. These two men are Naudin (1852) and Lecoq (1854).

Having discussed the early services of German and French nature-philosophy in establishing the doctrine of descent, we turn to the third great country of Europe, to free England, which during the last ten years has become the chief seat and starting-point for the further working out and definite establishment of the theory of development. Englishmen, who now take such an active part in every great scientific progress of humanity, and are the first to promote the eternal truths of natural science, at the beginning of the century took but little part in the continental nature-philosophy and its most important progress, the Theory of Descent. Almost the only earlier English naturalist whom we have here to mention is Erasmus Darwin, the grandfather of the reformer of the Theory of Descent. In 1795 he published, under the t.i.tle of "Zoonomia," a scientific work in which he expresses views very similar to those of Goethe and Lamarck, without, however, then knowing anything about these two men. It is evident that the Theory of Descent at that time pervaded the intellectual atmosphere.

Erasmus Darwin lays great stress upon the transformation of animal and vegetable species by their own vital action and by their becoming accustomed to changed conditions of existence, etc. Next, W. Herbert, in 1822, expressed the opinion that species of animals and plants are nothing but varieties which have become permanent. In like manner Grant, in Edinburgh, in 1826, declared that new species proceed from existing species by continued transformation. In 1841 Freke maintained that all organic beings must be descended from a single primitive type. In 1852 Herbert Spencer demonstrated minutely, and in a very clear and philosophic manner, the necessity of the Doctrine of Filiation, and established it more firmly in his excellent "Essays," which appeared in 1858, and in his "Principles of Biology," which was published at a later date. He has, at the same time, the great merit of having applied the theory of development to psychology, and of having shown that the emotional and intellectual faculties could only have been acquired by degrees and developed gradually. Lastly, we have to mention that in 1859 Huxley, the first of English zoologists, spoke of the Theory of Descent as the only hypothesis of creation reconcilable with scientific physiology. The same year produced the "Introduction to the Flora of Tasmania," in which Hooker, the celebrated English botanist, adopts the Theory of Descent, supporting it with important observations of his own.

All the naturalists and philosophers with whom we have become acquainted in this brief historical survey, as men adopting the Theory of Development, merely arrived at the conception that all the different species of animals and plants which at any time have lived, and still live, upon the earth, are the gradually changed and transformed descendants of one or some few original and very simple prototypes, which latter arose out of inorganic matter by spontaneous generation.

But none of them succeeded in placing this fundamental element of the doctrine of descent in relation with some cause, nor in satisfactorily explaining the transformation of organic species by the true demonstration of its mechanical antecedents. Charles Darwin was the first who solved this most difficult problem, and this forms the broad gulf which separates him from his predecessors.

The special merit of Charles Darwin is, in my opinion, twofold: in the first place, the doctrine of descent, the fundamental idea of which was already clearly expressed by Goethe and Lamarck, has been developed by him much more comprehensively, has been traced much more minutely in all directions, and carried out much more strictly and connectedly than by any of his predecessors; and secondly, he has established a new theory, which reveals to us the natural causes of organic development, the acting causes (causae efficientes) of organic form-production, and of the changes and transformations of animal and vegetable species. This is the theory which we call the Theory of Selection, or more accurately, the Theory of Natural Selection (selectio naturalis).

When we reflect that (with the few exceptions above mentioned) the whole science of Biology, before Darwin's time, was elaborated in accordance with the opposite views, and that almost all zoologists and botanists regarded the absolute independence of organic species as a self-evident inference from the results of all study of forms, we shall certainly not lightly value the twofold merit of Darwin. The false doctrine of the constancy and independent creation of individual species had gained such high authority, was so generally recognized, and was, moreover, so much favoured by delusive appearances, accepted by superficial observation, that, indeed, no small degree of courage, strength, and intelligence was required to rise as a reformer against its omnipotence, and to dash to pieces the structure artificially erected upon it. But, in addition to this, Darwin added to Lamarck's and Goethe's doctrine of descent the new and highly important principle of "natural selection."

We must sharply distinguish the two points-though this is usually not done-first, Lamarck's Theory of Descent, which only a.s.serts _that_ all animal and vegetable species are descended from common, most simple, and spontaneously generated prototypes; and secondly, Darwin's Theory of Selection, which shows us _why_ this progressive transformation of organic forms took place, and what causes, acting mechanically, effected the uninterrupted production of new forms, and the ever increasing variety of animals and plants.

Darwin's immortal merit cannot be justly estimated until a later period, when the Theory of Development, after overthrowing all other theories of creation, will be recognized as the supreme principle of explanation in Anthropology, and, consequently, in all other sciences. At present, while in the hot contest for truth the name of Darwin is the watchword to the advocates of the natural theory of development, his merits are inaccurately appreciated on both sides, for some persons overestimate them as much as others underestimate them.

His merit is overestimated when he is regarded as the founder of the Theory of Descent, or of the whole of the Theory of Development. We have seen from the historical sketch in this and the preceding chapters, that the Theory of Development, as such, is not new; all philosophers who have refused to be led captive by the blind dogma of a supernatural creation, have been compelled to a.s.sume a natural development. But the Theory of Descent const.i.tuting the specially biological part of the universal Theory of Development, had already been so clearly expressed by Lamarck, and carried out so fully by him to its most important consequences, that we must honour him as the real founder of it. Hence it is only the Theory of Selection, and not that of Descent, which may be called _Darwinism_; but this is in itself of so much importance, that its value can scarcely be overestimated.

Darwin's merit is naturally underestimated by all his opponents. But it is scarcely possible in this matter to point to scientific opponents, who are ent.i.tled by profound biological culture to p.r.o.nounce an opinion.

For among all the works opposed to Darwin and the Theory of Descent yet published, with the exception of that of Aga.s.siz, not one deserves consideration, much less refutation; all have so evidently been written either without thorough knowledge of biological facts, or without a clear philosophical understanding of the question in hand. We need not trouble ourselves at all about the attacks of theologians and other unscientific men, who really know nothing whatever of nature.

The only eminent scientific adversary who still remains opposed to Darwin and the whole theory of development is Louis Aga.s.siz; but the principle of his opposition in reality deserves notice only as a philosophical curiosity. In a French translation of his "Essay on Cla.s.sification,"(5) which we have spoken of before, published in Paris in 1869, Aga.s.siz has most formally announced his opposition to Darwinism, which he had previously expressed in many ways. To this translation he has appended a treatise of sixteen pages, bearing the t.i.tle, "Le Darwinisme. Cla.s.sification de Haeckel." This curious chapter contains the most wonderful things; as, for example, "Darwin's idea is a conception _a priori_. Darwinism is a burlesque of facts. Science would renounce the claim which it has. .h.i.therto possessed to the confidence of earnest minds if such sketches were to be accepted as indications of a true progress." The following pa.s.sage, however, is the climax of this strange polemic: "Darwinism shuts out almost the whole ma.s.s of acquired knowledge in order to retain and a.s.similate to itself that only which may serve its doctrine."

Surely this is what we may call turning the whole affair topsy-turvy!

The biologist who knows the facts must be astounded at Aga.s.siz's courage in uttering such sentences-sentences without a word of truth in them, and which he cannot himself believe! The impregnable strength of the Theory of Descent lies just in the fact that all biological facts are explicable only through it, and that without it they remain unintelligible miracles. All our "laborious knowledge" in comparative anatomy and physiology-in embryology and palaeontology-in the doctrine of the geographical and topographical distribution of organisms, etc., const.i.tutes an irrefutable testimony to the truth of the Theory of Descent.

In my General Morphology, especially in the sixth book (in the General Phylogeny), I have minutely refuted Aga.s.siz's "Essay on Cla.s.sification"

in all essential points. The twenty-fourth chapter I have devoted to a very detailed and strictly scientific discussion of that section which Aga.s.siz himself considers the most important (the groups or categories of systematic zoology and botany), and have shown that this part of his work is purely chimerical, without any trace of real foundation. Aga.s.siz takes good care not to venture anywhere to touch upon my refutation, because, forsooth, he is not in a position to produce anything substantial against it. He fights not with arguments, but with phrases.

However, such opposition will not delay the complete victory of the Theory of Development, but only accelerate it.

CHAPTER VI.

THEORY OF DEVELOPMENT ACCORDING TO LYELL AND DARWIN.

Charles Lyell's Principles of Geology.-His Natural History of the Earth's Development.-Origin of the Greatest Effects through the Multiplication of the Smallest Causes.-Unlimited Extent of Geological Periods.-Lyell's Refutation of Cuvier's History of Creation.-The Establishment of the Uninterrupted Connection of Historical Development by Lyell and Darwin.-Biographical Notice of Charles Darwin.-His Scientific Works.-His Theory of Coral Reefs.-Development of the Theory of Selection.-A Letter of Darwin's.-The Contemporaneous Appearance of Darwin's and Alfred Wallace's Theory of Selection.-Darwin's Study of Domestic Animals and Cultivated Plants.-Andreas Wagner's notions as to the Special Creation of Cultivated Organisms for the good of Man.-The Tree of Knowledge in Paradise.-Comparison between Wild and Cultivated Organisms.-Darwin's Study of Domestic Pigeons.-Importance of Pigeon Breeding.-Common Descent of all Races of Pigeons.

During the thirty years, from 1830 until 1859, when Darwin's work appeared, the ideas of creation introduced by Cuvier remained predominant in the sciences of organic nature. People rested satisfied with the unscientific a.s.sumption, that in the course of the earth's history, a series of inexplicable revolutions had periodically annihilated the whole world of animals and plants, and that at the end of each revolution, and the beginning of a new period, a new enlarged, and improved edition of the organic population had appeared. Although the number of these editions of creation was altogether problematical, and in truth could not be fixed at all, and although the numerous advances which, during this time, were made in all the departments of zoology and botany demonstrated more and more that Cuvier's hypothesis was unfounded and untenable, and that Lamarck's natural theory of development was nearer the truth, yet the former maintained its authority almost universally among biologists. This must, above all, be ascribed to the veneration which Cuvier had acquired, and strikingly ill.u.s.trates how injurious to the progress of humanity a faith in any definite authority may become. Authority, as Goethe once admirably said, perpetuates the individual, which as an individual should pa.s.s away, rejects and allows to pa.s.s that which should be held fast, and is the main obstacle to the advance of humanity.

It is only by having regard to the great weight of Cuvier's authority, and to the mighty potency of human indolence, which is with difficulty induced to depart from the broad and comfortable way of everyday conceptions, and to enter upon new paths not yet made easy, that we can comprehend how it is that Lamarck's Theory of Descent did not gain its due recognition until 1859, after Darwin had given it a new foundation.

The soil had long been prepared for it by the works of Charles Lyell, another English naturalist, whose views are of great importance for the natural history of creation, and must accordingly here be briefly explained.

In 1830 Charles Lyell published, under the t.i.tle of "Principles of Geology," a work in which he thoroughly reformed the science of Geology and the history of the earth's development, and effected this reform in a manner similar to that in which, thirty years later, Darwin in his work reformed the science of Biology. Lyell's great treatise, which radically destroyed Cuvier's hypothesis of creation, appeared in the same year in which Cuvier celebrated his triumph over the nature-philosophy, and established his supremacy in the domain of morphology for the following thirty years. Whilst Cuvier, by his artificial hypothesis of creation and his theory of catastrophes connected with it, directly obstructed the path of the theory of natural development, and cut off all chance of a natural explanation, Lyell once more opened a free road, and brought forward convincing geological evidence to show that Cuvier's dualistic conceptions were as unfounded as they were superfluous. He demonstrated that those changes of the earth's surface, which are still taking place before our eyes, are perfectly sufficient to explain everything we know of the development of the earth's crust in general, and that it is superfluous and useless to seek for mysterious causes in inexplicable revolutions. He showed that we need only have recourse to the hypothesis of exceedingly long periods of time in order to explain the formation of the crust of the earth in the simplest and most natural manner by means of the very same causes which are still active. Many geologists had previously imagined that the highest chains of mountains which rise on the surface of the earth could owe their origin only to enormous revolutions transforming a great part of the earth's surface, especially to colossal volcanic eruptions. Such chains of mountains as those of the Alps or the Cordilleras were believed to have arisen direct from the fiery fluid of the interior of the earth, through an enormous chasm in the broken crust. Lyell, on the other hand, showed that we can explain the formation of such enormous chains of mountains quite naturally by the same slow and imperceptible risings and depressions of the earth's surface which are still continually taking place, and the causes of which are by no means miraculous. Although these depressions and risings may perhaps amount only to a few inches, or at most a few feet, in the course of a century; still, in the course of some millions of years they are perfectly sufficient to raise up the highest chains of mountains, without the aid of mysterious and incomprehensible revolutions. In like manner, the meteorological action of the atmosphere, the influence of rain and snow, and, lastly, the breakers on the coasts, which by themselves seem to produce an insignificant effect, must cause the greatest changes if we only allow sufficiently long periods for their action. The multiplication of the smallest causes produces the greatest effects.

Drops of water produce a cavity in a rock.

I shall afterwards be obliged again to recur to the immeasurable length of geological periods which are necessary for this purpose, for, as we shall see, Darwin's theory, as well as that of Lyell, renders the a.s.sumption of immense periods absolutely necessary. If the earth and its organisms have actually developed in a natural way, this slow and gradual development must certainly have taken a length of time which surpa.s.ses our powers of comprehension. But as many men see in this very circ.u.mstance one of the princ.i.p.al difficulties in the way of those theories of development, I beg leave here to remark that we have not a single rational ground for conceiving the time requisite to be limited in any way. Not only many ordinary persons, but even eminent naturalists, make it their chief objection to these theories, that they arbitrarily claim too great a length of time: yet the ground of objection is scarcely intelligible. For it is absolutely impossible to see what can, in any way, limit us in a.s.suming long periods of time. We have long known, even from the structure of the stratified crust of the earth alone, that its origin and the formation of neptunic rocks from water must have taken, at least, several millions of years. From a strictly philosophical point of view, it makes no difference whether we hypothetically a.s.sume for this process ten millions or ten thousand billions of years. Before us and behind us lies eternity. If the a.s.sumption of such enormous periods is opposed to the feelings of many, I regard this simply as the consequence of false notions which are impressed upon us from our earliest youth concerning the short history of the earth, which is said to embrace only a few thousands of years.

Albert Lange, in his "History of Materialism,"(12) has convincingly shown that from a strictly philosophical point of view it is far less objectionable in a scientific hypothesis to a.s.sume periods which are too long than periods which are too short. Every process of development is the more intelligible the longer it is a.s.sumed to last. A short and limited period is the most improbable.

I have no s.p.a.ce here to enter minutely into Lyell's great work, and will therefore mention only its most important result, which is, that he completely refuted Cuvier's history of creation with its mythical revolutions, and established in its place the constant and slow transformation of the earth's crust by the continued action of forces, which are still working on the earth's surface, viz., the movement of water and the volcanic fluid of the interior of earth. Lyell thus demonstrated a continuous and uninterrupted connection of the whole history of the earth, and he proved it so irrefutably, and established so convincingly the supremacy of the "existing causes," that is, of the causes which are still active in the transformation of the earth's crust, that Geology in a short time completely renounced Cuvier's hypothesis.

Now, it is remarkable that Palaeontology, the science of petrifactions, so far as it was pursued by botanists and zoologists, remained apparently unaffected by this great progress in geology. Biology still continued to a.s.sume repeated new creations of the whole animal and vegetable kingdoms, at the beginning of every new period of the earth's history, although this hypothesis of individual creations, shoved into the world one after the other, without the a.s.sumption of Cuvier's cataclysms, became pure nonsense, and lost its foundation. It is evidently perfectly absurd to a.s.sume a distinct new creation of the whole world of animals and plants at definite epochs, without the crust of the earth itself experiencing any considerable general revolution.

And although this conception is most closely connected with Cuvier's theory of catastrophes, still it prevailed when the latter had been completely destroyed and abandoned.

It was reserved for the great English naturalist, Charles Darwin, to remove this contradiction, and to show that the organic beings of the earth have a history as continuous and connected as the inorganic crust of the earth; that animals and plants have arisen from one another by as gradual a trans.m.u.tation as that by which the varying forms of the earth's crust, the forms of the continents, and of the seas surrounding and separating them, have arisen out of earlier and quite different forms. In this respect we may truly say that in the domain of Zoology and Botany Darwin made the same progress as Lyell, his great countryman, in the domain of Geology. Both proved the _uninterrupted connection of the historical development_, and demonstrated a gradual trans.m.u.tation of the different conditions succeeding one another.

The special merit of Darwin, as I have already remarked in a preceding chapter, is twofold. In the first place, he has treated the Theory of Descent, put forth by Lamarck and Goethe, in a much more comprehensive manner, as a whole, and carried it out in a much more connected manner, than had been done by any one of his predecessors. Secondly, he has established the causal foundation of this Theory of Descent by the Theory of Selection, which is peculiarly his own; that is, he has demonstrated the acting _causes of the changes_ which the Theory of Descent simply stated, as _facts_. The Theory of Descent, introduced into Biology in 1809, by Lamarck, a.s.serts that all the different species of animals and plants are descended from a single or some few most simple prototypes, produced by spontaneous generation. The Theory of Selection, established in 1859 by Darwin, shows us _why_ this must be so; it points out the acting causes in a manner with which Kant would have been delighted, and indeed, in the domain of organic nature, Darwin has become the Newton whose advent Kant thought himself ent.i.tled prophetically to deny.

Now, before we approach Darwin's theory, it will perhaps be of interest to notice a few details as to the personal character of this great naturalist, as to his life, and the way in which he was led to form his doctrine. Charles Robert Darwin was born at Shrewsbury, on the Severn, on the 12th of February, 1809; therefore, at present he is sixty-three years old. In his seventeenth year (1825) he entered the University of Edinburgh, and two years later Christ's College, Cambridge. When scarcely twenty-two years old, in 1831, he was invited to take part in a scientific expedition which was sent out by England, in order to survey accurately the southernmost point of South America, and to examine several parts of the South Seas. This expedition, like many other voyages of inquiry fitted out in a praiseworthy manner by England, had scientific objects, and at the same time was intended to solve practical problems relating to navigation. The vessel, commanded by Captain Fitzroy, appropriately bore the symbolic name of the _Beagle_. The voyage of the _Beagle_, which lasted five years, was of the highest importance to the full development of Darwin's genius; for in the very first year, when he set his foot on the soil of South America, the outline of the doctrine of descent dawned upon him. Darwin himself has described this voyage in a work which is written in a very attractive style, and the perusal of which I strongly recommend to the reader. This book of travel, which lies far above the usual average in interest, not only shows in a very charming manner Darwin's amiable character, but we can in many ways recognize the various steps by which he arrived at his conceptions. The result of the voyage was, first, a large scientific work, the zoological and geological portion of which belong in a great measure to Darwin; and secondly, a celebrated work by him alone on Coral Reefs, which in itself would have sufficed to secure to him a lasting reputation. It is well known that the islands in the South Seas consist for the most part of coral reefs, and are surrounded by them. Formerly no satisfactory explanation could be given of their different and remarkable forms, and of their relation to those islands which are not formed of corals. It was reserved for Darwin to solve this difficult problem, for together with the constructive action of the coral zoophytes, he a.s.sumed geological risings and depressions of the bottom of the sea to account for the origin of the different forms of reefs.

Darwin's Theory of the Origin of Coral Reefs, like his later one as to the Origin of Organic Species, is a theory which fully explains the phenomenon, and for this purpose a.s.sumes only the simplest natural causes, without hypothetically supporting it with any unknown processes.

Among the remaining works of Darwin, I must not pa.s.s over his excellent monograph on Cirrhipedia, a curious cla.s.s of marine animals, which in their outward appearance resemble mussels, and were actually considered by Cuvier as Molluscs possessing two sh.e.l.ls, while in truth they belonged to the Crustacea (crabs).

The extraordinary hardships to which Darwin had been exposed during his voyage in the _Beagle_ had injured his health to such a degree, that after his return home he was obliged to withdraw from the restless turmoil of London life, and since then has lived in quiet retirement on his estate at Down, near Bromley, in Kent. This seclusion from the restless activity of the great city certainly exercised a beneficial influence upon Darwin, and it is probable that we owe to it, at least partially, the formation of the Theory of Selection. Undisturbed by the various engagements which in London would have wasted his strength, he was enabled to concentrate his attention upon the great problem to which his mind had been turned during his voyage in the _Beagle_. In order to show what kind of observations during the voyage princ.i.p.ally gave rise to the fundamental idea of the Theory of Selection, and in what manner he afterwards worked it out, I shall insert here a pa.s.sage from a letter which he addressed to me on the 8th of October, 1864.

_Letter from Charles Darwin to Haeckel, 8th October, 1864._

"In South America three cla.s.ses of facts were brought strongly before my mind. _Firstly_, the manner in which closely allied species replace species in going southward. _Secondly_, the close affinity of the species inhabiting the islands near South America to those proper to the continent. This struck me profoundly, especially the difference of the species in the adjoining islets in the Galopagos Archipelago. _Thirdly_, the relation of the living Edentata and Rodentia to the extinct species.

I shall never forget my astonishment when I dug out a gigantic piece of armour like that of the living armadillo.

"Reflecting on these facts, and collecting a.n.a.logous ones, it seemed to me probable that allied species were descended from a common parent. But for some years I could not conceive how each form became so excellently adapted to its habits of life. I then began systematically to study domestic productions, and after a time saw clearly that man's selective power was the most important agent. I was prepared, from having studied the habits of animals, to appreciate the struggle for existence, and my work in geology gave me some idea of the lapse of past time. Therefore, when I happened to read "Malthus on Population," the idea of natural selection flashed on me. Of all the minor points, the last which I appreciated was the importance and cause of the principle of divergence."