Vlll INTRODUCTION. 



tree of the animal kingdom where the main branches emerge from the darkness 

 of the pre- Cambrian period; it will furnish us the only means by which we can 

 hope to solve some of the most important problems in vertebrate morphology, 

 such as the meaning of vertebrate cephalogenesis, of concrescence, germ layers, 

 gastrulation, and the structure of the oldest fossil representatives of the vertebrate 

 series. The answer to such problems cannot be found till after we have dis- 

 covered the immediate ancestors of the vertebrates and the broad outlines of their 

 structure, for when the point of departure is determined, and only then, can we 

 determine which is the base and which is the summit of a series of changes, which 

 the primitive, which the derived; in short, the direction in which evolution is 

 moving. 



The arachnid theory of the origin of vertebrates has made slow progress. 

 This is not surprising since it has had to contend against the fixed ideas of the 

 specialist working in some narrow field of vertebrate or invertebrate morph- 

 ology, and who is unfamiliar with the multitude of facts and details, intricate in 

 themselves and in their bearings, upon which the arachnid theory rests. It has 

 also had to contend against the indifference of the newer school of biologists, who 

 look on morphology as an exhausted field, and who attach an exaggerated import- 

 ance to experimental, or statistical work, or to the minute structure of cells, or to 

 the analysis of protoplasm. 



This is largely due to a common misconception of the real aim of the mor- 

 phologist; for it is evident that tracing the identity of structure under the disguise 

 of new forms is only the beginning of the morphologist's work. His real problem 

 is to measure the rate of these changes, and to seek out the underlying causes. 

 Hence, a great morphological problem, such as the origin of vertebrates, is essen- 

 tially a problem in experimental evolution, an experiment performed on the largest 

 scale of any in the history of organic evolution. But here the problem presents 

 itself in a different form from the ordinary laboratory experiment. There the ex- 

 perimenter fixes the conditions, as nearly as possible, and then records and meas- 

 ures the events as they appear. Here the morphologist records and measures the 

 events, and from them tries to discover the conditions. I believe I have discovered 

 the main events in this experiment of Nature, and I have recorded it, in terms of 

 systematic zoology, in a genealogic tree of the great arthropod-vertebrate stock. 

 This discovery enables us to see clearly some of the factors that have brought 

 about the results. They are mainly internal factors, insignificant in themselves, 

 but acquiring such immense transforming power by persistent and prolonged • 

 action that it is unnecessary to invoke the agency of such factors as external 

 environment, natural selection, and heredity. At most, it seems to me, these 

 factors can account only for the superficial details of the essentially completed 

 body. Morphology teaches us that the foundations of anatomical structure are 

 automatically created by the processes of growth and organic readjustment, and 

 that they remain essentially unmoved by external conditons. 



For almost a quarter of a century the problem of the evolution of the verte- 



