Nothing but good can, in my opinion, result from the incursion of the non-specialist into the realm of the specialist, provided that the former is in earnest. Over and over again the chemist has given valuable help to the physicist, and the physicist to the chemist, so closely allied are the two subjects; so also is it with physiology and anatomy, the two subjects are so interdependent that a worker in the one may give valuable aid towards the solution of some large problem which is the special territory of the other. It has been a matter of surprise to many how it came about that {2}I, a worker in the physiological laboratory at Cambridge ever since Foster introduced experimental physiology into English-speaking nations, should have devoted so much time to the promulgation of a theory of the origin of vertebrates--a subject remote from physiology, and one of the larger questions appertaining to comparative anatomy. By what process of thought was I led to take up the consideration of a subject apparently so remote from all my previous work, and so foreign to the atmosphere of a physiological laboratory? It may perhaps be instructive to my readers to see how one investigation leads to another, until at last, _nolens volens_, the worker finds himself in front of a possible solution to a problem far removed from his original investigation, which by the very magnitude and importance of it forces him to devote his whole energy and time to seeing whether his theory is good. In the years 1880-1884 I was engaged in the investigation of the action of the heart, and the nature of the nerves which regulate that action. In the course of that investigation I was struck by the ease with which it was possible to distinguish between the fibres of the vagus and accelerator nerves on their way to the heart, owing to the medullation of the former and the non-medullation of the latter. This led me to an investigation of the accelerator fibres, to find out how far they are non-medullated, and so to the discovery that the _rami communicantes_ connecting together the central nervous system and the sympathetic are in reality single, not double, as had hitherto been thought; for the grey _ramus communicans_ is in reality a peripheral nerve which supplies the blood-vessels of the spinal cord and its membranes, and is of the same nature as the grey accelerators to the heart.