GROWTH OF NEW ORGANS.

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with comparative anatomy and the history of development will find as little difficulty about the origin of completely new organs as about the utter disappearance of rudimentary organs. The disappearance of the latter, viewed by itself, is the converse of the origin of the former. Both processes are particular phenomena of differentiation, which, like all others, can be explained quite simply and mechanically by the action of natural selection in the struggle for life.

When we closely examine the first appearance of new organs, we as a rule observe nothing more than an increased growth of one part of some already existing organ. But as this part undertakes other functions, in accordance with the laws of division of labour and the change of labour, a separation soon becomes evident which leads to the gradual development of the new organ, in accordance with the theory of selection. This development is determined both by the physiological laws of growth and nutrition, as also happens in the reverse case, in that of retrogression in rudimentary organs.

The infinite importance of the study of rudimentary organs for the fundamental questions of natural philosophy cannot be too highly estimated (see chap. xix. of my "General Morphology," p. 266); we might set up with their aid a theory of the unsuitability of parts in organisms, as a counter-hypothesis to the old popular doctrine of the suitability of parts. This latter dualistic teleology finally leads us to supernatural dogmas and miracles, whereas we obtain from the former, monistic dysteleology, a firm foundation for our mechanical interpretation of nature; it leads us, by means of teleological mechanism, to pure Monism (see Chap. XIV.).

CHAPTER XIII.

THE

THE INDIVIDUAL DEVELOPMENT OF ORGANISMS. HISTORY OF THE DEVELOPMENT OF THE ANIMAL TRIBES.

General Importance of Individual Development (Ontogeny).-Defects of our Present Education.-Facts in the Individual Development.— Agreement in the Individual Development of Man and the Vertebrate Animals.-The Human Egg.-Fertilization.-Immortality.-The Cleavage of the Egg.-Formation of Germ-layers.-Gastrulation.— History of the Development of the Central Nervous System, of the Extremities, of the Branchial Arches and of the Tail in Vertebrate Animals. Causal Connection between Ontogenesis and Phylogenesis.— The Fundamental Law of Biogenesis.-Palingenesis or Recapitulative Development.-Cenogenesis or Disordered Development.-Stages in Comparative Anatomy.-Its Relation to the Paleontological and Embryological Series of Development.

THE greater number of educated persons who nowadays show more or less interest in our theories of development unfortunately know next to nothing of the facts of organic development from actual observation. Man, like other mammals, appears at birth in an already developed form. The chicken, like other birds, creeps out of the egg in a completely developed form. But the wonderful processes by which these completed animal forms arise are entirely unknown to most persons. And yet these but little con

sidered processes contain a fund of knowledge, which is unsurpassed by any other in general importance. For we here have the development before our eyes as a tangible fact, and we need only place a number of hen's eggs in an incubator, and watch their development for three weeks carefully with a microscope, in order to understand the mystery by which a highly organized bird develops out of a single simple cell. Step by step we can trace this wonderful transformation, and step by step point out how one organ is developed out of the other.

And for this reason alone-because, in fact, it is in this domain only that the facts of development are presented to us in tangible reality. I consider it of paramount consequence to direct the reader's careful attention to those infinitely important and interesting processes in the individual development of organisms, viz. to ontogeny, and above all to the ontogeny of the vertebrate animals, including man. I wish specially to recommend these exceedingly remarkable and instructive phenomena to the reader's most careful consideration; for, on the one hand, they form one of our strongest supports for the theory of descent, and for the monistic conception of the universe generally; and, on the other hand, because hitherto it is only a few privileged persons who have properly estimated their immense general importance. These phenomena will be found discussed very fully in my "Anthropogeny."

We cannot, indeed, but be astonished when we consider the deep ignorance which still prevails, in the widest circles, about the facts of the individual development of man and organisms in general. These facts, the universal importance of which cannot be estimated too highly, were established,

in their most important outlines, even more than a hundred years ago, in 1759, by the great German naturalist Caspar Friedrich Wolff, in his classical "Theoria Generationis." But, just as Lamarck's Theory of Descent, founded in 1809, lay dormant for half a century, and was only awakened to new and imperishable life in 1859, by Darwin, in like manner Wolff's Theory of Epigenesis remained unknown for nearly half a century; and it was only after Oken, in 1806, had published his history of the development of the intestinal tube, and after Meckel, in 1812, had translated Wolff's work (written in Latin) on the same subject into German, that Wolff's theory of epigenesis became more generally known, and has since formed the foundation of all subsequent investigations of the history of individual development. The study of ontogenesis thus received a great stimulus, and soon there appeared the classical investigations of the two friends, Christian Pander (1817) and Carl Ernst Bär (1819). Bär, in his remarkable “Entwickelungsgeschichte der Thiere," 20 worked out the ontogeny of vertebrate animals in all its important facts. He carried out a series of such excellent observations, and illustrated them by such profound philosophical reflections, that his work became the foundation for a thorough understanding of this important group of animals, to which, of course, man also belongs. The facts of embryology alone would be sufficient to solve the question of man's position in nature, which is the highest of all problems. Look attentively at and compare the eight figures which are represented on the adjoining Plates II. and III., and it will be seen that the philosophical importance of embryology cannot be too highly estimated.