8 GENERAL PHYSIOLOGY 



the organic world. The notion of the derivation of man from animal- 

 like ancestors originally inhabiting the water, is found clearly ex- 

 pressed by Anaximander (b. about 620 B.C.); and Heraclitus (about 

 500 B.C.) had an idea of the significance of the struggle for existence 

 (epts). But the theory of Empedocles (b. 504 B.C.) upon the origin 

 of living things is the clearest and most surprising. According to 

 him, plants appeared first, then the lower animals, and from them 

 the higher animals and, finally, men were developed by a process 

 of perfection. The effective principle in this perfecting process he 

 perceived in the fact that ill-adapted individuals are destroyed in 

 the struggle for life, while those that are capable of living produce 

 offspring. Almost twenty-five hundred years elapsed before this 

 simple conception of the descent and natural selection of organisms, 

 clearly expressed by Empedocles, was empirically grounded by 

 Darwin and was established as the natural explanation of the 

 otherwise marvellous multiplicity of organic forms. 



Many ideas, more or less correct, regarding special physiological 

 phenomena are found also among the early Greek philosophers. 

 But these scattered truths are mingled with so many fantastic and 

 purely arbitrary notions that, from their associations, they lose their 

 real value. No coherent, systematic observations or reflections 

 concerning vital phenomena exist before Aristotle. 



From the side of practical medicine, likewise, the investigation 

 of life experienced no considerable advance, even when medical art, 

 hitherto without a critic, was placed by Hippocrates (460-377 B.c.) 

 upon a sound basis. 



A physiological doctrine appeared first among the followers of 

 Hippocrates, probably under the influence of Plato's philosophy, 

 and it was soon perfected and controlled all the medical ideas of 

 that time. This is the doctrine of the spirits (trvevfia), in the main 

 thought of which can be found the first germ of a fundamental 

 physiological truth. This doctrine asserts that the pneuma, an 

 excessively subtile material agent, is attracted by the human lungs, 

 passes from the lungs into the blood, and is distributed by the 

 latter throughout the body. All vital phenomena depend upon 

 the action of this agent. This conception, which, naturally, was 

 adorned with all sorts of absurd accompaniments, suggests strongly 

 our modern ideas concerning the role of oxygen in the organism. 



B. THE PERIOD OF GALEN 



The first intimation of an attempt to explain vital phenomena 

 appears in the early Hippocratic doctrine of the pneuma. This 

 was expanded, especially in the Alexandrian school, by Herophilus 

 (about 300 B.C.) and Erasistratus (d. 280 B.C.), the latter of whom 

 distinguished a 7rvev/j,a ^COTIKOV (vital spirits) in the heart and a 

 (animal spirits) in the brain. From this it is 



