31 THE HISTORY OF BIOLOGY 



them as being the only really existing element, Plato finds true reality in 

 the world of abstract thought and maintains that what is perceptible by 

 the senses is an imperfect image of the eternal ideal, the divine intelli- 

 gence, conceivable only by abstract thinking. The nearer things are to 

 the divinity, the more perfect and animate do they become. Thus the 

 stars of heaven possess a higher animate life than man and are created in 

 greater likeness to the supreme intelligence. Of man the first thing to be 

 created was the head, which Plato, like Democritus, regarded as the organ 

 of the animate soul; the head has very nearly the spherical form corre- 

 sponding to the ideal, and trunk and limbs are created to save the head the 

 trouble of rolling along the ground. In the trunk dwells a lower, mortal 

 soul, whose best part, the heart, the organ of courage, is separated by the 

 diaphragm from the organ of animal desires, the digestive apparatus, the 

 most vital part of which, the liver, however, has the merit of producing 

 dreams during sleep, from which the future can be predicted. The plants on 

 our earth are created to provide man with food; animals, again, are sprung 

 from men whose souls have degenerated and have consequently been given 

 an inferior dwelling; first, women have come into being out of cowardly 

 men's souls, then birds and quadrupeds out of such human beings as have 

 neglected their intelligence, and, finally, the most worthless souls have been 

 placed in aquatic animals, "which may not even breathe pure air." This 

 theory of the migration of the soul is clearly reminiscent of that of Pythag- 

 oras; from him, too, no doubt originates Plato's mystical theory of number, 

 the details of which do not belong to our present subject. 



Systematt%ation of thought 

 It may seem unnecessary to dwell so long on these fantasies. They are, how- 

 ever, well worth noting, for their originator has exercised a rare and radi- 

 cal influence on human culture in its entirety and even, as we shall find later 

 on, no small influence upon the development of biological science. His philo- 

 sophical speculations related above have in this respect had but little signif- 

 icance. Plato's greatest contribution has been made in the sphere of the 

 purely ideal intellectual life. The spirit of man is, he said, bound by laws far 

 more abiding than those that may be deduced from natural phenomena. To 

 these inflexible standards for intellectual activity he is led by speculations 

 in the ethical sphere, in which, following Socrates, he found a definite paral- 

 lel between human actions and their consequences. Having advanced thus 

 far, he devoted himself to developing this theory of the conditions, bound 

 by immutable law, of the world of ideas, in which, as mentioned above, he 

 saw the true existence, of which the phenomena visible in natural life are 

 mere images. Everything on this earth, then, has its eternal idea as its proto- 

 type; every individual horse is an imperfect image of the idea horse, which 

 is eternal and perfect. Through this reasoning Plato came to be the founder 



