It took a very long time before the simple and natural view was accepted, namely, that petrifactions are in reality nothing but what they appear to simple observation-the indestructible remains of extinct organisms. It is true the celebrated painter, Leonardo da Vinci, in the 15th century, ventured to a.s.sert that the mud which was constantly deposited by water was the cause of petrifactions, as it surrounded the indestructible sh.e.l.ls of mussels and snails which lay at the bottom of the waters, and gradually turned them into solid stone. The same idea was maintained in the 16th century by a Parisian potter, Palissy by name, who became celebrated on account of his invention of china.

However, the so-called "professional men" were very far from paying any regard to these correct a.s.sertions of a simple and healthy human understanding; it was not till the end of the last century that it was generally accepted, in consequence of the foundation of the Neptunian geology by Werner.

The foundation of a more strictly scientific palaeontology, however, belongs to the beginning of our century, when Cuvier published his cla.s.sic researches on petrified Vertebrate animals, and when his great opponent, Lamarck, made known his remarkable investigations on fossil Invertebrate animals, especially on petrified snails and clams. In Cuvier's celebrated work "On the Fossil Bones" of Vertebrate animals-princ.i.p.ally of mammals and reptiles-we see that he had already arrived at the knowledge of some very important and general palaeontological laws, which are of great consequence to the history of creation. Foremost among them stands the a.s.sertion that the extinct species of animals, whose remains we find petrified in the different strata of the earth's crust, lying one above another, differ all the more strikingly from the still living kindred species of animals the deeper those strata lie-in other words, the earlier the animals lived in past ages. In fact, in every perpendicular section of the stratified crust of the earth we find that the different strata, deposited by the water in a certain historical succession, are characterized by different petrifactions, and that these extinct organisms become more like those of the present day the higher the strata lie; in other words, the more recent the period in the earth's history in which they lived, died, and became encrusted by the deposited and hardened strata of mud.

However important this general observation of Cuvier's was in one sense, yet in another it became to him the source of a very serious error. For as he considered the characteristic petrifactions of each individual group of strata (which had been deposited during one main period of the earth's history) to be entirely different from those of the strata lying above or below, and as he erroneously believed that one and the same species of animal was never found in two succeeding groups of strata, he arrived at the false idea, which was accepted as a law by most subsequent naturalists, that a series of quite distinct periods of creation had succeeded one another. Each period was supposed to have had its special animal and vegetable world, each its peculiar specific Fauna and Flora.

Cuvier imagined that the whole history of the earth's crust, since the time when living creatures had first appeared on the surface, must be divided into a number of perfectly distinct periods, or divisions of time, and that the individual periods must have been separated from one another by peculiar revolutions of an unknown nature (cataclysms, or catastrophes). Each revolution was followed by the utter annihilation of the till then existing animals and plants, and after its termination a completely new creation of organic forms took place. A new world of animals and plants, absolutely and specifically distinct from those of the preceding historical periods, was called into existence at once, and now again peopled the globe for thousands of years, till it again perished suddenly in the crash of a new revolution.

About the nature and causes of these revolutions, Cuvier expressly said that no idea could be formed, and that the present active forces in nature were not sufficient for their explanation. Cuvier points out four active causes as the natural forces, or mechanical agents, at present constantly but slowly at work in changing the earth's surface: first, _rain_, which washes down the steep mountain slopes and heaps up debris at their foot; secondly, _flowing waters_, which carry away this debris and deposit it as mud in stagnant waters; thirdly, the sea, whose breakers gnaw at the steep _sea_ coasts, and throw up "dunes" on the flat sea margins; finally and fourthly, _volcanos_, which break through and heave up the strata of the earth's hardened crust, and pile up and scatter about the products of their eruptions. Whilst Cuvier recognizes the constant slow transformation of the present surface of the earth by these four mighty causes, he a.s.serts at the same time that they would not have sufficed to effect the revolutions of the remote ages, and that the anatomical structure of the earth's surface cannot be explained by the necessary action of those mechanical agents: the great and marvellous revolutions of the whole earth's surface must, according to him, have been rather the effects of very peculiar causes, completely unknown to us; the usual thread of development was broken by them, and the course of nature altered.

These views Cuvier explained in a special work "On the Revolutions of the Earth's Surface, and the Changes which they have wrought in the Animal World." They were maintained, and generally accepted for a long time, and became the greatest obstacle to the development of a natural history of the creation. For if such all-destructive revolutions had actually occurred, of course a continuity of the development of species, a connecting thread in the organic history of the earth, could not be admitted at all, and we should be obliged to have recourse to the action of supernatural forces; that is, to the interference of miracles in the natural course of things. It is only through miracles that these revolutions of the earth could have been brought about, and it is only through miracles that, after their cessation and at the commencement of each new period, a new animal and vegetable kingdom could have been created. But science has no room for miracles, for by miracles we understand an interference of supernatural forces in the natural course of development of matter.

Just as the great authority which Linnaeus gained by his system of distinguishing and naming organic species led his successors to a complete ossification, as it were, of the dogmatic idea of species and to a real abuse of the systematic distinction implied by it, so the great services which Cuvier had rendered to the knowledge and distinction of extinct species became the cause of a general adoption of his theory of revolutions and catastrophes, and of the false views of creation connected therewith. The consequence of this was that, during the first half of our century, most zoologists and botanists clung to the opinion that a series of independent periods in the organic history of the earth had existed; that each period was distinguished by distinct and peculiar kinds of animal and vegetable species; that these were annihilated at the termination of the period by a general revolution; and that, after the cessation of the latter, a new world of different species of animals and plants was created.

It is true some independent thinkers, above all the great physical philosopher, Lamarck, even at an early period, set forth a series of weighty reasons which refuted Cuvier's theory of cataclysms, and pointed to a perfectly continuous and uninterrupted developmental history of all the organic inhabitants of the earth through all ages.

They maintained that the animal and vegetable species of each period were derived from those of the preceding period, and were only the altered descendants of the former. This true conception, however, being opposed to Cuvier's great authority, was then unable to make way. Nay, even after Cuvier's theory of catastrophes had been completely cast out from the domain of geology by Lyell's cla.s.sic Principles of Geology, which appeared in 1830, still his idea of the specific distinctness of a series of organic creations maintained its influence, in many ways, in the science of Palaeontology. (Gen. Morph. ii. 312.)

By a curious coincidence, thirteen years ago, almost at the same time that Cuvier's History of Creation received its death-blow by Darwin's book, another celebrated naturalist made an attempt to re-establish it, and to adopt it in the roughest manner, as a part of a teleologico-theological system of nature. This was the Swiss geologist, Louis Aga.s.siz, who attained a great reputation by his theory of glaciers and the ice-period, borrowed from Schimper and Charpentier, and who has been living in North America for many years. He commenced in 1858 to publish a work planned on a very large scale, which bears the t.i.tle of "Contributions to the Natural History of the United States of North America." The first volume of this work, although large and costly, owing to the patriotism of the Americans, had an unprecedented sale; its t.i.tle is, "An Essay on Cla.s.sification."(5)

In this essay Aga.s.siz not only discusses the natural series of organisms, and the different attempts of naturalists at cla.s.sification, but also all the general biological phenomena which have reference to it. The history of the development of organisms, both the embryonal and the palaeontological, comparative anatomy, the general economy of nature, the geographical and topographical distribution of animals and plants-in short, almost all the general phenomena of organic nature are discussed in Aga.s.siz's Essay on Cla.s.sification, and are explained in a sense and from a point of view which is thoroughly opposed to that of Darwin. While Darwin's chief merit lies in the fact that he demonstrates natural causes for the coming into existence of animal and vegetable species, and thereby establishes the mechanical or monistic view of the universe as regards this most difficult branch of the history of creation, Aga.s.siz, on the contrary, strives to exclude every mechanical hypothesis from the subject, and to put the supernatural interference of a personal Creator in the place of the natural forces of matter; consequently, to establish a thoroughly teleological or dualistic view of the universe. It will not be out of place if I examine a little more closely Aga.s.siz's biological views, and especially his ideas of creation, because no other work of our opponents treats the important fundamental questions with equal minuteness, and because the utter untenableness of the dualistic conception of nature becomes very evident from the failure of this attempt.

The organic _species_, the various conceptions of which we have above designated as the real centre of dispute in the opposed views of creation, is looked upon by Aga.s.siz, as by Cuvier and Linnaeus, as a form unchangeable in all its essential characteristics. The species may indeed change and vary within certain narrow limits; never in essential qualities, but only in unessential points. No new species could ever proceed from the changes or varieties of a species. Not one of all organic species, therefore, is ever derived from another, but each individual species has been separately created by G.o.d. Each individual species, as Aga.s.siz expresses it, is "an embodied creative thought" of G.o.d.

In direct opposition to the fact established by palaeontological experience, that the duration of the individual organic species is most unequal, and that many species continue unchanged through several successive periods of the earth's history, while others only existed during a small portion of such a period, Aga.s.siz maintains that one and the same species never occurs in two different periods, but that each individual period is characterized by species of animals and plants which are quite peculiar, and belong to it exclusively. He further shares Cuvier's opinion that the whole of these inhabitants were annihilated by the great and universal revolutions of the earth's surface, which divide two successive periods, and that after its destruction a new and specifically different a.s.semblage of organisms was created. This new creation Aga.s.siz supposes to have taken place in this manner: viz., that at each creation all the inhabitants of the earth, in their full average number of individuals, and in the peculiar relations corresponding to the economy of nature, were, as a whole, suddenly placed upon the earth by the Creator. In saying this he puts himself in opposition to one of the most firmly established and most important laws of animal and vegetable geography-namely, to the law that each species has a single original locality of origin, or a so-called "centre of creation," from which it has gradually spread over the rest of the earth. Instead of this, Aga.s.siz a.s.sumes each species to have been created at several points of the earth's surface, and that in each case a large number of individuals was created.

The "natural system" of organisms, the different groups and categories of which arranged above one another-namely, the branches, cla.s.ses, orders, families, genera, and species-we consider, in accordance with the Theory of Descent, as different branches and twigs of the organic family-tree, is, according to Aga.s.siz, the direct expression of the divine plan of creation, and the naturalist, while investigating the natural system, repeats the creative thoughts of G.o.d. In this Aga.s.siz finds the strongest proof that man is the image and child of G.o.d. The different stages of groups or categories of the natural system correspond with the different stages of development which the divine plan of creation had attained. The Creator, in projecting and carrying out this plan, starting from the most general ideas of creation, plunged more and more into specialities. For instance, when creating the animal kingdom, G.o.d had in the first place four totally distinct ideas of animal bodies, which he embodied in the different structures of the four great, princ.i.p.al forms, types, or branches of the animal kingdom; namely, vertebrate animals, articulate animals, molluscous animals, and radiate animals. The Creator then, having reflected in what manner he might vary these four different plans of structure, next created within each of the four princ.i.p.al forms, several different cla.s.ses-for example, in the vertebrate animal form, the cla.s.ses of mammals, birds, reptiles, amphibious animals, and fishes. Then G.o.d further reflected upon the individual cla.s.ses, and by various modifications in the structure of each cla.s.s, he produced the individual orders. By further variation in the order, he created natural families. As the Creator further varied the peculiarities of structure of individual parts in each family, genera arose. In further meditation on his plan of creation, he entered so much into detail that individual species came into existence, which, consequently, are embodied creative thoughts of the most special kind. It is only to be regretted that the Creator expressed these most special and most deeply considered "creative thoughts" in so very indistinct and loose a manner, and that he imprinted so vague a stamp upon them, and permitted them to vary so freely that not one naturalist is able to distinguish the "good" from the "bad species," or a genuine species from varieties, races, etc.

(Gen. Morph. ii. 373.)

We see, then, according to Aga.s.siz's conception, that the Creator, in producing organic forms, goes to work exactly as a human architect, who has taken upon himself the task of devising and producing as many different buildings as possible, for the most manifold purposes, in the most different styles, in various degrees of simplicity, splendour, greatness, and perfection. This architect would perhaps at first choose four different styles for all these buildings, say the Gothic, Byzantine, Chinese, and Rococo styles. In each of these styles he would build a number of churches, palaces, garrisons, prisons, and dwelling-houses. Each of these different buildings he would execute in ruder and more perfect, in greater and smaller, in simpler and grander fashion, etc. However, the human architect would perhaps, in this respect, be better off than the divine Creator, as he would have perfect liberty in the number of graduated subordinate groups. The Creator, however, according to Aga.s.siz, can only move within six groups or categories: the species, genus, family, order, cla.s.s, and type. More than these six categories do not exist for him.

When we read Aga.s.siz's book on cla.s.sification, and see how he carries out and establishes these strange ideas, we can scarcely understand how, with all the appearance of scientific earnestness, he can persevere in his idea of the divine Creator as a man-like being (anthropomorphism), for by his explanation of details he produces a picture of the most absurd nonsense. In the whole series of these suppositions the Creator is nothing but an all-mighty man, who, plagued with _ennui_, amuses himself with planning and constructing most varied toys in the shape of organic species. After having diverted himself with these for thousands of years, they become tiresome to him, he destroys them by a general revolution of the earth's surface, and thus throws the whole of the useless toys in heaps together; then, in order to while away his time with something new and better, he calls a new and more perfect animal and vegetable world into existence. But in order not to have the trouble of beginning the work of creation over again, he keeps, in the main, to his original plan of creation, and creates merely new species, or at most only new genera, and much more rarely new families, new orders, or cla.s.ses. He never succeeds in producing a new style or type, and always keeps strictly within the six categories or graduated groups.

When, according to Aga.s.siz, the Creator has thus amused himself for thousands of millions of years with constructing and destroying a series of different creations, at last (but very late) he is struck with the happy thought of creating something like himself, and so makes man in his own image. The end of all the history of creation is thus arrived at and the series of revolutions of the earth is closed. Man, the child and image of G.o.d, gives him so much to do, causes him so much pleasure and trouble, that he is wearied no longer, and therefore need not undertake a new creation. It is clear that if, according to Aga.s.siz, we once a.s.sign to the Creator entirely human attributes and qualities, and regard his work of creation as entirely a.n.a.logous to human creative activity, we are necessarily obliged to admit such utterly absurd inferences as those just stated.

The many intrinsic contradictions and perversities in Aga.s.siz's view of creation-a view which necessarily led him to the most decided opposition to the Theory of Descent-must excite our astonishment all the more because, in his earlier scientific works, he had in many respects actually paved the way for Darwin, especially by his researches in Palaeontology. Among the numerous investigations which created general interest in the then young science of Palaeontology, those of Aga.s.siz, especially his celebrated work on "Fossil Fish," rank next in importance to Cuvier's work, which formed the foundation of the science. The petrified fish, with which Aga.s.siz has made us acquainted, have not only an extremely great importance for the understanding of all groups of Vertebrate animals, and their historical development, but we have arrived through them at a sure knowledge of important general laws of development, some of which were first discovered by Aga.s.siz. He it was who drew special attention to the remarkable parallelism between the embryonal and the palaeontological development-between ontogeny and phylogeny, which I have already (p. 10) claimed as one of the strongest pillars of the Theory of Descent. No one before had so distinctly stated as Aga.s.siz did, that, of the Vertebrate animals, fishes alone existed, at first, that amphibious animals came later, and that birds and mammals appeared only at a much later period, further, that among mammals, as among fishes, imperfect and lower orders had appeared first, but more perfect and higher orders at a later period. Aga.s.siz, therefore, showed that the palaeontological development of the whole Vertebrate group was not only parallel with the embryonic, but also with the systematic development, that is, with the graduated series which we see everywhere in the system, ascending from the lower to the higher cla.s.ses, orders, etc.

In the earth's history lower forms appeared first, the higher forms later. This important fact, as well as the agreement of the embryonic and palaeontological development, is explained quite simply and naturally by the Doctrine of Descent, and without it is perfectly inexplicable.

This cause holds good also in the great law of _progressive development_, that is, of the historical progress of organization, which is traceable, broadly and as a whole, in the historical succession of all organisms, as well as in the special perfecting of individual parts of animal bodies. Thus, for example, the skeleton of Vertebrate animals acquired at first slowly, and by degrees, that high degree of perfection which it now possesses in man and the other higher Vertebrate animals.

This progress, acknowledged in point of fact by Aga.s.siz, necessarily follows from Darwin's Doctrine of Descent, which demonstrates its active causes. If this doctrine is correct, the perfecting and diversification of animal and vegetable species must of necessity have gradually increased in the course of the organic history of the earth, and could only attain its highest perfection in most recent times.

The above-mentioned laws of development, together with some other general ones, which have been expressly admitted and justly emphasized by Aga.s.siz, and some of which have first been set forth by him, are, as we shall see later, only explicable by the Theory of Descent, and without it remain perfectly incomprehensible. The conjoint action of Inheritance and Adaptation, as explained by Darwin, can alone be their true cause. But they all stand in sharp and irreconcilable opposition to the hypothesis of creation maintained by Aga.s.siz, as well as to the idea of a personal Creator who acts for a definite purpose. If we seriously wish to explain those remarkable phenomena and their inter-connection by Aga.s.siz's theory, then we are necessarily driven to the curious supposition that the Creator himself has developed, together with the organic nature which he created and modelled. We can, in that case, no longer rid ourselves of the idea that the Creator himself, like a human being, designed, improved, and finally, with many alterations, carried out his plans. "Man grows as higher grow his aims," and the same supposition, so unworthy of a G.o.d, must be applied to him. Although, from the reverence with which, in every page, Aga.s.siz speaks of the Creator, it might appear that, on his theory, we attain to the sublimest conception of the divine activity in nature, yet the contrary is in truth the case. The divine Creator is degraded to the level of an idealized man, of an organism progressing in development!

Considering the wide popularity and great authority which Aga.s.siz's work has gained, and which is perhaps justified on account of earlier scientific services rendered by the author, I have thought it my duty here to show the utter untenableness of his general conceptions. So far as this work pretends to be a scientific history of creation, it is undoubtedly a complete failure. But still it has great value, being the only detailed attempt, adorned with scientific arguments, which an eminent naturalist of our day has made to found a teleological or dualistic history of creation. The utter impossibility of such a history has thus been made obvious to every one. No opponent of Aga.s.siz could have refuted the dualistic conception of organic nature and its origin more strikingly than he himself has done by the intrinsic contradictions which present themselves everywhere in his theory.

The opponents of the monistic or mechanical conception of the world have welcomed Aga.s.siz's work with delight, and find in it a perfect proof of the direct creative action of a personal G.o.d. But they overlook the fact that this personal Creator is only an idealized organism, endowed with human attributes. This low dualistic conception of G.o.d corresponds with a low animal stage of development of the human organism. The more developed man of the present day is capable of, and justified in, conceiving that infinitely n.o.bler and sublimer idea of G.o.d which alone is compatible with the monistic conception of the universe, and which recognizes G.o.d's spirit and power in all phenomena without exception.

This monistic idea of G.o.d, which belongs to the future, has already been expressed by Giordano Bruno in the following words:-"A spirit exists in all things, and no body is so small but contains a part of the divine substance within itself, by which it is animated." It is of this n.o.ble idea of G.o.d that Goethe says:-"Certainly there does not exist a more beautiful worship of G.o.d than that which needs no image, but which arises in our heart from converse with Nature." By it we arrive at the sublime idea of the Unity of G.o.d and Nature.

CHAPTER IV.

THEORY OF DEVELOPMENT ACCORDING TO GOETHE AND OKEN.

Scientific Insufficiency of all Conceptions of a Creation of Individual Species.-Necessity of the Counter Theories of Development.-Historical Survey of the Most Important Theories of Development.-Aristotle.-His Doctrine of Spontaneous Generation.-The Meaning of Natural Philosophy.-Goethe.-His Merits as a Naturalist.-His Metamorphosis of Plants.-His Vertebral Theory of the Skull.-His Discovery of the Mid Jawbone in Man.-Goethe's Interest in the Dispute between Cuvier and Geoffroy St. Hilaire.-Goethe's Discovery of the Two Organic Formative Principles, of the Conservative Principle of Specification (by Inheritance), and of the Progressive Principle of Transformation (by Adaptation).-Goethe's Views of the Common Descent of all Vertebrate Animals, including Man.-Theory of Development according to Gottfried Reinhold Trevira.n.u.s.-His Monistic Conception of Nature.-Oken.-His Natural Philosophy.-Oken's Theory of Protoplasm.-Oken's Theory of Infusoria (Cell Theory).-Oken's Theory of Development.

All the different ideas which we may form of a separate and independent origin of the individual organic species by creation lead us, when logically carried out, to a so-called _anthropomorphism_, that is, to imagining the Creator as a man-like being, as was shown in our last chapter. The Creator becomes an organism who designs a plan, reflects upon and varies this plan, and finally forms creatures according to this plan, as a human architect would his building. If even such eminent naturalists as Linnaeus, Cuvier, and Aga.s.siz, the princ.i.p.al representatives of the dualistic hypothesis of creation, could not arrive at a more satisfactory view, we may take it as evidence of the insufficiency of all those conceptions which would derive the various forms of organic nature from a creation of individual species.

Some naturalists, indeed, seeing the complete insufficiency of these views, have tried to replace the idea of a personal Creator by that of an unconsciously active and creative Force of Nature; yet this expression is evidently merely an evasive phrase, as long as it is not clearly shown what this force of nature is, and how it works. Hence these attempts, also, have been absolute failures. In fact, whenever an independent origin of the different forms of animals and plants has been a.s.sumed, naturalists have found themselves compelled to fall back upon so many "acts of creation," that is, on supernatural interferences of the Creator in the natural course of things, which in all other cases goes on without interference.

It is true that several teleological naturalists, feeling the scientific insufficiency of a supernatural "_creation_," have endeavoured to save the hypothesis by wishing it to be understood that creation "is nothing else than a way of coming into being, unknown and inconceivable to us."

The eminent Fritz Muller has cut off from this sophistic evasion every chance of escape by the following striking remark:-"It is intended here only to express in a disguised manner the shamefaced confession, that they neither have, nor care to have, _any opinion_ about the origin of species. According to this explanation of the word, we might as well speak of the creation of cholera, or syphilis, of the creation of a conflagration, or of a railway accident, as of the creation of man."

(Jenaische Zestscrift, bd. v. p. 272.)

In the face, then, of these hypotheses of creation, which are scientifically insufficient, we are forced to seek refuge in the _counter-theory of development_ of organisms, if we wish to come to a rational conception of the origin of organisms. We are forced and obliged to do so, even if the theory of development only throws a glimmer of probability upon a mechanical, natural origin of the animal and vegetable species; but all the more if, as we shall see, this theory explains all facts simply and clearly, as well as completely and comprehensively. The theories of development are by no means, as they often falsely are represented to be, arbitrary fancies, or wilful products of the imagination, which only attempt approximately to explain the origin of this or that individual organism; but they are theories founded strictly on science, which explain in the simplest manner, from a fixed and clear point of view, the whole of organic natural phenomena, and more especially the origin of organic species, and demonstrate them to be the necessary consequences of mechanical processes in nature.

As I have already shown in the second chapter, all these theories of development coincide naturally with that general theory of the universe which is usually designated as the uniform or _monistic_, often also as the _mechanical_ or causal, because it only a.s.sumes mechanical causes, or _causes working by necessity_ (causae efficientes), for the explanation of natural phenomena. In like manner, on the other hand, the supernatural hypotheses of creation which we have already discussed coincide completely with the opposite view of the universe, which in contrast to the former is called the twofold or _dualistic_, often the _teleological_ or vital, because it traces the organic natural phenomena to final causes, acting and _working for a definite purpose_ (causae finales). It is this deep and intrinsic connection of the different theories of creation with the most important questions of philosophy that incites us to their closer examination.

The fundamental idea, which must necessarily lie at the bottom of all natural theories of development, is that of a _gradual development of all (even the most perfect) organisms_ out of a single, or out of a very few, quite simple, and quite imperfect original beings, which came into existence, not by supernatural creation, but by _spontaneous generation_, or archigony, out of inorganic matter. In reality, there are two distinct conceptions united in this fundamental idea, but which have, nevertheless, a deep intrinsic connection-namely, first, the idea of spontaneous generation (or archigony) of the original primary beings; and secondly, the idea of the progressive development of the various species of organisms from those most simple primary beings. These two important mechanical conceptions are the inseparable fundamental ideas of every theory of development, if scientifically carried out. As it maintains the derivation of the different species of animals and plants from the simplest, common primary species, we may term it also the Doctrine of Filiation, or _Theory of Descent_; as there is also a change of species connected with it, it may also be termed the _Trans.m.u.tation Theory_.

While the supernatural histories of creation must have originated thousands of years ago, in that very remote primitive age when man, first developing out of the monkey-state, began for the first time to think more closely about himself, and about the origin of the world around him, the natural theories of development, on the other hand, are necessarily of much more recent origin. These views are met with only among nations of a more matured civilization, to whom, by philosophic culture, the necessity of a knowledge of natural causes has become apparent; and even among these, only individual and specially gifted natures can be expected to have recognized the origin of the world of phenomena, as well as its course of development, as the necessary consequences of mechanical, naturally active causes. In no nation have these preliminary conditions, for the origin of a natural theory of development, ever existed in so high a degree as among the Greeks of cla.s.sic antiquity. But, on the other hand, they lacked a close acquaintance with the facts of the processes and forms of nature, and, consequently, the foundation based upon experience, for a satisfactory unravelling of the problem of development. Exact investigation of nature, and the knowledge of nature founded on an experimental basis, was of course almost unknown to antiquity, as well as to the Middle Ages, and is only an acquisition of modern times. We have therefore here no special occasion to examine the natural theories of development of the various Greek philosophers, since they were wanting in the knowledge gained by experience, both of organic and inorganic nature, and since they almost always, as the consequence, lost themselves in airy speculations.

One man only must be mentioned here by way of exception,-Aristotle, the greatest and the only truly great naturalist of antiquity and the Middle Ages, one of the grandest geniuses of all time. To what a degree he stands there alone, during a period of more than two thousand years, in the region of empirico-philosophical knowledge of nature, and especially in his knowledge of organic nature, is proved to us by the precious remains of his but partially surviving works. In them many traces are found of a theory of natural development. Aristotle a.s.sumes, as a matter of certainty, that spontaneous generation was the natural manner in which the lower organic creatures came into existence. He describes animals and plants originating from matter itself, through its own original force; as, for example, moths from wool, fleas from putrid dung, wood-lice from damp wood, etc. But as the distinction of organic species, which Linnaeus only arrived at two thousand years later, was unknown to him, he could form no ideas about their genealogical relations.

The fundamental notion of the theory of development, that the different species of animals and plants have been developed from a common primary species by transformation, could of course only be clearly a.s.serted after the kinds of species themselves had become better known, and after the extinct species had been carefully examined and compared with the living ones. This was not done until the end of the last and the beginning of the present century. It was not until the year 1801 that the great Lamarck expressed the theory of development, which he, in 1809, further elaborated in his cla.s.sical "Philosophie Zoologique."

While Lamarck and his countryman, Geoffroy St. Hilaire, in France, opposed Cuvier's views, and maintained a natural development of organic species by transformation and descent, Goethe and Oken at the same time pursued the same course in Germany, and helped to establish the theory of development. As these naturalists are generally called nature-philosophers (Naturphilosophen), and as this ambiguous designation is correct in a certain sense, it appears to me appropriate here to say a few words about the correct estimate of the "Natur-philosophie."

Although for many years in England the ideas of natural science and philosophy have been looked upon as almost equivalent, and as every truly scientific investigator of nature is most justly called there a "natural philosopher," yet in Germany for more than half a century natural science has been kept strictly distinct from philosophy, and the union of the two into a true philosophy of nature is recognized only by the few. This misapprehension is owing to the fantastic eccentricities of earlier German natural-philosophers, such as Oken, Sch.e.l.ling, etc.; they believed that they were able to construct the laws of nature in their own heads, without being obliged to take their stand upon the grounds of actual experience. When the complete hollowness of their a.s.sumptions had been demonstrated, naturalists, in "the nation of thinkers," fell into the very opposite extreme, believing that they would be able to reach the high aim of science, that is, the knowledge of truth, by the mere experience of the senses, and without any philosophical activity of thought.

From that time, but especially since 1830, most naturalists have shown a strong aversion to any general, philosophical view of nature. The real aim of natural science was now supposed to consist in the knowledge of details, and it was believed that this would be attained in the study of biology, when the forms and the phenomena of life, in all individual organisms, had become accurately known, by the help of the finest instruments and means of observation. It is true that among these strictly empirical, or so-called exact naturalists, there were always very many who rose above this narrow point of view, and sought the final aim in a knowledge of the general laws of organization. Yet the great majority of zoologists and botanists, during the thirty or forty years preceding Darwin, refused to concern themselves about such general laws; all they admitted was, that perhaps in the far distant future, when the end of all empiric knowledge should have been arrived at, when all individual animals and plants should have been thoroughly examined, naturalists might begin to think of discovering general biological laws.

If we consider and compare the most important advances which the human mind has made in the knowledge of truth, we shall soon see that it is always owing to philosophical mental operations that these advances have been made, and that the experience of the senses which certainly and necessarily precedes these operations, and the knowledge of details gained thereby, only furnish the basis for those general laws.

Experience and philosophy, therefore, by no means stand in such exclusive opposition to each other as most men have hitherto supposed; they rather necessarily supplement each other. The philosopher who is wanting in the firm foundation of sensuous experience, of empirical knowledge, is very apt to arrive at false conclusions in his general speculations, which even a moderately informed naturalist can refute at once. On the other hand, the purely empiric naturalists, who do not trouble themselves about the philosophical comprehension of their sensuous experiences, and who do not strive after general knowledge, can promote science only in a very slight degree, and the chief value of their hard-won knowledge of details lies in the general results which more comprehensive minds will one day derive from them.

From a general survey of the course of biological development since Linnaeus' time, we can easily see, as Bar has pointed out, a continual vacillation between these two tendencies, at one time a prevalence of the empirical-the so-called exact-and then again of the philosophical or speculative tendency. Thus at the end of the last century, in opposition to Linnaeus' purely empirical school, a natural-philosophical reaction took place, the moving spirits of which, Lamarck, Geoffroy St.

Hilaire, Goethe, and Oken, endeavoured by their mental work to introduce light and order into the chaos of the acc.u.mulated empirical raw material. In opposition to the many errors and speculations of these natural philosophers, who went too far, Cuvier then came forward, introducing a second, purely empirical period. It reached its most one-sided development between the years 1830-1860, and there now followed a second philosophical reaction, caused by Darwin's work. Thus during the last ten years, men again have begun to endeavour to obtain a knowledge of the general laws of nature, to which, after all, all detailed knowledge of experience serves only as a foundation, and through which alone it acquires its true value. It is through philosophy alone that natural knowledge becomes a true science, that is, a philosophy of nature. (Gen. Morph. i. 63-108.)

Jean Lamarck and Wolfgang Goethe stand at the head of all the great philosophers of nature who first established a theory of organic development, and who are the ill.u.s.trious fellow-workers of Darwin. I turn first to our beloved Goethe, who, among all, stands in the closest relations to us Germans. However, before I explain his special services to the theory of development, it seems to me necessary to say a few words about his importance as a naturalist in general, as it is commonly very little known.

I am sure most of my readers honour Goethe only as a poet and a man; only a few have any conception of the high value of his scientific works, and of the gigantic stride with which he advanced before his own age-advanced so much that most naturalists of that time were unable to follow him. In several pa.s.sages of his scientific writings he bitterly complains of the narrow-mindedness of professed naturalists, who do not know how to value his works (who cannot see the wood for the trees), and who cannot rouse themselves to discover the general laws of nature among the ma.s.s of details. He is only too just when he utters the reproach-"The philosophers will very soon discover that observers rarely rise to a stand-point from which they can survey so many important objects." It is true, at the same time, that their want of appreciation was caused by the false road into which Goethe was led in his theory of colours.

This theory of colours, which he himself designates as the favourite production of his leisure, however much that is beautiful it may contain, is a complete failure in regard to its foundations. The exact mathematical method by means of which alone it is possible, in inorganic sciences, but above all in physics, to raise a structure step by step on a thoroughly firm basis, was altogether repugnant to Goethe. In rejecting it he allowed himself not only to be very unjust towards the most eminent physicists, but to be led into errors which have greatly injured the fame of his other valuable works. It is quite different in the organic sciences, in which we are but rarely able to proceed, from the beginning, upon a firm mathematical basis; we are rather compelled, by the infinitely difficult and intricate nature of the problem, at the first to form inductions-that is, we are obliged to endeavour to establish general laws by numerous individual observations, which are not quite complete. A comparison of kindred series of phenomena, or the method of combination, is here the most important instrument for inquiry, and this method was applied by Goethe with as much success as with conscious knowledge of its value, in his works relating to the philosophy of nature.

The most celebrated among Goethe's writings relating to organic nature is his _Metamorphosis of Plants_, which appeared in 1790, a work which distinctly shows a grasp of the fundamental idea of the theory of development, inasmuch as Goethe, in it, was labouring to point out a single organ, by the infinitely varied development and metamorphosis of which the whole of the endless variety of forms in the world of plants might be conceived to have arisen; this fundamental organ he found in the _leaf_. If at that time the microscope had been generally employed, if Goethe had examined the structure of organisms by the means of the microscope he would have gone still further, and would have seen that the leaf is itself a compound of individual parts of a lower order, that is, of _cells_. He would then not have declared that the leaf, but that the _cell_ is the real fundamental organ by the multiplication, transformation, and combination (synthesis) of which, in the first place, the leaf is formed; and that, in the next place, by transformation, variation, and combination of leaves there arise all the varied beauties in form and colour which we admire in the green parts, as well as in the organs of propagation, or the flowers of plants. Goethe here showed that in order to comprehend the whole of the phenomena, we must in the first place compare them, and, secondly, search for a simple type, a simple fundamental form, of which all other forms are only infinite variations.

Something similar to what he had here done for the metamorphosis of plants he then did for the Vertebrate animals, in his celebrated _vertebral theory of the skull_. Goethe was the first to show, independently of Oken, who almost simultaneously arrived at the same thought, that the skull of man and of all Vertebrate animals, in particular mammals, is nothing more than a bony case, formed of the same bones,-that is, vertebrae,-out of which the spine also is composed. The vertebrae of the skull are like those of the spine, bony rings lying behind each other, but in the skull are peculiarly changed and specialized (differentiated). Although this idea has been strongly modified by recent discoveries, yet in Goethe's day it was one of the greatest advances in comparative anatomy, and was not only one of the first advances towards the understanding of the structure of Vertebrate animals, but at the same time explained many individual phenomena. When two parts of a body, such as the skull and spine, which appear at first sight so different, were proved to be parts originally the same, developed out of one and the same foundation, one of the difficult problems of the philosophy of nature was solved. Here again we meet the notion of a single type-the conception of a single principle, which becomes infinitely varied in the different species, and in the parts of individual species.