The law of recapitulation--Vindication of this law by the theory advanced in this book--The germ-layer theory--Its present position-- A physiological not a morphological conception--New fundamental law required--Composition of adult body--Neuro-epithelial syncytium and free-living cells--Meaning of the blastula--Derivation of the Metazoa from the Protozoa--Importance of the central nervous system for Ontogeny as well as for Phylogeny--Derivation of free-living cells from germ-cells--Meaning of coelom--Formation of neural canal--Gastrula of Amphioxus and of Lucifer--Summary 455 CHAPTER XV FINAL REMARKS Problems requiring investigation-- Giant nerve-cells and giant nerve-fibres; their comparison in fishes and arthropods; blood- and lymph-corpuscles; nature of the skin; origin of system of unstriped muscles; origin of the sympathetic nervous system; biological test of relationship. Criticisms of Balanoglossus theory--Theory of parallel development-- Importance of the theory advocated in this book for all problems of Evolution 488 BIBLIOGRAPHY AND INDEX OF AUTHORS 501 GENERAL INDEX 517 "_GO ON AND PROSPER; THERE IS NOTHING SO USEFUL IN SCIENCE AS ONE OF THOSE EARTHQUAKE HYPOTHESES, WHICH OBLIGE ONE TO FACE THE POSSIBILITY THAT THE SOLIDEST-LOOKING STRUCTURES MAY COLLAPSE._" LETTER FROM PROF. HUXLEY TO THE AUTHOR. JUNE 2, 1889. {1}THE ORIGIN OF VERTEBRATES _INTRODUCTION_ In former days it was possible for a man like Johannes Müller to be a leader both in physiology and in comparative anatomy. Nowadays all scientific knowledge has increased so largely that specialization is inevitable, and every investigator is confined more and more not only to one department of science, but as a rule to one small portion of that department. In the case of such cognate sciences as physiology and comparative anatomy this limiting of the scope of view is especially deleterious, for zoology without physiology is dead, and physiology in many of its departments without comparative anatomy can advance but little. Then, again, the too exclusive study of one subject always tends to force the mind into a special groove--into a line of thought so deeply tinged with the prevalent teaching of the subject, that any suggestions which arise contrary to such teaching are apt to be dismissed at once as heretical and not worthy of further thought; whereas the same suggestion arising in the mind of one outside this particular line of thought may give rise to new and valuable scientific discoveries.