THE DOGMA OF EVOLUTION 



blood; although its full significance was not appre- 

 ciated until much later, yet the knowledge that the 

 blood made a complete circuit in the body, was what 

 was needed to link biology to the physical sciences. 

 And since then, it has been more and more persistent- 

 ly the aim of biologists to emphasize the mechanical 

 functions of the living body, to point out its analo- 

 gies to a machine, and to minimize the unknown 

 causes which still pass under the name of vitality. 



It was the universally accepted belief that organic 

 bodies were composed of the four Aristotelian ele- 

 ments — earth, water, air, and fire; by the operation 

 of the active principles of heat and moisture, the ele- 

 ments were metamorphosed into three primary sub- 

 stances, called salt, sulphur, and mercury, which in 

 their proper and balanced proportions make up the 

 frame and tissue of living bodies. The character of 

 the individual was assumed to be determined by four 

 humours which composed the fluid portion of the 

 body; it is not clear how these were related to the 

 primary elements and substances, as they seem in 

 some way to have involved the principle of life. The 

 humours were classified as blood, bile, phlegm, and 

 water, and are still currently used to characterize the 

 disposition as phlegmatic, melancholic, etc. Both 

 health and character were held to depend on the right 

 balance of the three substances and four humours ; it 

 was the business of the physician to determine the 

 disturbance, and by the aid of herbs, magic, and 



