Dominican friar, Giordano Bruno, did so even more explicitly. The latter was burnt at the stake for this, by the Christian inquisition in Rome, on the 17th of Feb., 1600, on the same day on which, 36 years before, Galileo, his great fellow-countryman and fellow-worker, was born. On the Campo di Fiori in Rome, where that funeral pile once stood, free Italy a short time ago (in July, 1889) unveiled a monument erected to the memory of the great martyr of the monistic theory; an eloquent sign of the immense change which time has wrought.

By the Theory of Descent we are for the first time enabled to conceive of the unity of nature in such a manner that a mechanico-causal explanation of even the most intricate organic phenomena, for example, the origin and structure of the organs of sense, is no more difficult (in a general way) than is the mechanical explanation of any physical process; as, for example, earthquakes, the courses of the wind, or the currents of the ocean. We thus arrive at the extremely important conviction that all natural bodies which are known to us are equally animated, that the distinction which has been made between animate and inanimate bodies does not exist. When a stone is thrown into the air, and falls to earth according to definite laws, or when in a solution of salt a crystal is formed, or when sulphur and quicksilver unite in forming cinnabar, the phenomenon is neither more nor less a mechanical manifestation of life than the growth and flowering of plants, than the propagation of animals or the activity of their senses, than the perception or the formation of thought in man. The forces of nature present themselves here merely in different combinations and forms, sometimes simpler, some

times more complex. Bound elasticities become free and pass over into living forces, or vice versa. This restoration of the monistic conception of nature constitutes the chief and most comprehensive merit of our new theory of development, and is the crown of modern natural science.

CHAPTER II.

SCIENTIFIC JUSTIFICATION OF THE THEORY OF DEHISTORY OF CREATION ACCORDING TO

SCENT.

LINNÆUS.

The Theory of Descent, or Doctrine of Filiation, as the Monistic Explanation of Organic Natural Phenomena.-Its Comparison with Newton's Theory of Gravitation.-Limits of Scientific Explanation and of Human Knowledge in general.—All Knowledge founded originally on Sensuous Experience, à posteriori.-Transition of à posteriori Knowledge, by Inheritance, into à priori Knowledge.-Contrast between the Supernatural Hypotheses of the Creation according to Linnæus, Cuvier, Agassiz, and the Natural Theories of Development according to Lamarck, Goethe, and Darwin.-Connection of the former with the Monistic (mechanical), of the latter with the Dualistic Conception of the Universe.-Monism and Materialism.-Scientific and Moral Materialism.-The History of Creation according to Moses.-Linnæus as the Founder of the Systematic Description of Nature and Distinction of Species.-Linnæus' Classification and Binary Nomenclature.-Meaning of Linnæus' Idea of Species. -His History of Creation.-Linnæus' View of the Origin of Species. THE value which every scientific theory possesses is measured by the number and importance of the objects which can be explained by it, as well as by the simplicity and universality of the causes which are employed in it as grounds of explanation. On the one hand, the greater the number and the more important the meaning of the phenomena explained by the theory, and the simpler, on ⚫ the other hand, and the more general the causes which the

theory assigns as explanations, the greater is its scientific

value, the more safely we are guided by it, and the more strongly are we bound to adopt it.

Let us call to mind, for example, that theory which has ranked up to the present time as the greatest achievement of the human mind-the Theory of Gravitation, which Newton, two hundred years ago, established in his Mathematical Principles of Natural Philosophy. Here we find that the object to be explained is as large as one can well imagine. He undertook to reduce the phenomena of the motion of the planets, and the structure of the universe, to mathematical laws. As the most simple cause of these intricate phenomena of motion, Newton established the law of weight or attraction, the same law which is the cause of the fall of bodies, of adhesion, cohesion, and many other phenomena.

If we apply the same standard of valuation to Darwin's theory, we must arrive at the conclusion that this theory, also, is one of the greatest achievements of the human mind, and that it may be placed quite on a level with Newton's Theory of Gravitation. Perhaps this opinion will seem a little exaggerated, or at any rate very bold, but I hope in the course of this treatise to convince the reader that this estimate is not too high. In the preceding chapter, some of the most important and most general phenomena in organic nature, which have been explained by Darwin's theory, have been named. Among them are the variations in form which accompany the individual development of organisms, most varied and complicated phenomena, which until now presented the greatest difficulties in the way of mechanical explanation, that is, in the tracing of them to active causes. We have mentioned the rudimentary organs,

those exceedingly remarkable structures in animals and plants which have no object and refute every teleological explanation seeking for the final purpose of the organism. A great number of other phenomena might have been mentioned, which are no less important, and are explained in the simplest manner by Darwin's reformed Theory of Descent. For the present I will only mention the phenomena presented to us by the geographical distribution of animals and plants on the surface of our planet, as well as the geological distribution of the extinct and petrified organisms in the different strata of the earth's crust. These important palæontological and geographical phenomena, which were formerly only known to us as facts, are now traced to their active causes by the Theory of Descent.

The same statement applies further to all the general laws of Comparative Anatomy, especially to the great law of division of labour or separation (polymorphism, or differentiation), a law which determines the form or structure of human society, as well as the organization of individual animals and plants. It is this law which necessitates an ever increasing variety, as well as a progressive development of organic forms. This law of the division of labour has, up to the present time, been only recognized as a fact, and it, like the law of progressive development, or the law of progress which we perceive active everywhere in the history of nations (as also in that of animals and plants), is explained by Darwin's Doctrine of Descent. Then, if we turn our attention to the great whole of organic nature, if we compare all the individual groups of phenomena of this immense domain of life, it cannot fail to appear, in the light of the Doctrine of Descent, no longer as the ingeniously