298 GENERAL PHYSIOLOGY 



A. THEORIES CONCERNING THE ORIGIN OF LIFE UPON THE EARTH 



1. The Doctrine of Spontaneous Generation 



The modern doctrine of spontaneous generation (archegony, 

 abiogenesis, gejieratio fipontcmea or cequivoca, etc.) in its general 

 form is as follows. Since there was a time in the evolution of 

 the earth when the existence of the living substance that now 

 inhabits the cool surface of the latter was absolutely impossible, 

 living substance must have arisen from lifeless substance at some 

 later period. 



The question accordingly arises, how and under what conditions 

 were the first organisms created ? 



To the ancients, even to a mind having so comprehensive a 

 knowledge of' nature as that of Aristotle, the idea presented no 

 especial difiSculties that animals, such as worms, insects and even 

 fishes, could come into existence out of mud. Only at a relatively 

 late time and particularly in connection with the researches of 

 Redi and Swammerdamm upon the development of insects, were 

 these crude ideas laid aside as incompatible with established 

 scientific knowledge. 



But the doctrine of sjDontaneous generation obtained a new 

 point of support, when the invention of the microscope led to the 

 discovery of a world hitherto wholly unknown and excessively 

 rich in forms, when it was found that whenever an aqueous 

 infusion of dead organic substance was prepared, after a short time 

 an abundance of minute living beings developed in it, which even 

 yet are termed Infusoria. It was fully believed that in Infusoria 

 organisms had been found that were produced by spontaneous 

 generation out of the dead substances in the infusion. This view 

 necessarily seemed all the more probable because the Infusoria 

 were the lowest and simplest beings that had been known up to 

 that time. But in this case also it was established later that the 

 organisms did not originate spontaneously, but were developed 

 from germs that were previously contained in the substances or 

 came into the vessel through the air. Milne Edwards, Schwann, 

 Max Schultze, Helmholtz and others showed that if the substances 

 had previously been freed from germs by boiling, and if germs 

 were prevented from entering through the air, the development 

 of Infusoria never took place, however long the infusion was 

 allowed to stand. 



When, later, the smallest of all micro-organisms, the Bacteria, 

 began to attract strongly the attention of the scientific world, 

 and when it was found by refined methods of investigation that 

 these minute beings or their germs are present everywhere in the 

 air, the earth and the water, the doctrine of spontaneous generation 



