ch. viii.] THE BEGINNINGS OF LIFE. 419 



little was definitely known about the relations of organisms 

 to one another and to the inorganic world. Accordingly 

 with the very beginnings of modern biological knowledge, 

 and with the somewhat more cautious and systematic em- 

 ployment of induction characteristic of the seventeenth 

 century, the old belief in spontaneous generation was called 

 in question. By a series of very simple but apt experiments, 

 in which pieces of decaying meat were protected from 

 maggots by a gauze covering, the illustrious Redi proved, to 

 the satisfaction of everyone, that the maggots are not pro- 

 duced from the substance of the meat, but from eggs de- 

 posited therein by flies. So conclusive were these experi- 

 ments that the belief in spontaneous generation, which bad 

 hitherto rested chiefly upon phenomena of this sort, was 

 almost universally abandoned, and the doctrine that every 

 living thing comes from some living thing — omne vivum ex 

 vivo — received that general acceptance which it was destined 

 to retain down to the present time. With the progress of 

 biological knowledge, as the complex structures and regular 

 modes of growth of the lower animals began to be better 

 understood, and as the microscope began to disclose the 

 existence of countless forms of life infinitesimal in size but 

 complicated in organization, many of which were proved to 

 be propagated either by fission or by some kind of germina- 

 tion, the doctrine omne vivum ex vivo became more and more 

 implicitly regarded as a prime article of faith, and the hypo- 

 thesis of spontaneous generation was not merely scouted as 

 absurd, but neglected as unworthy of notice. 



Philosophical theories conspired with observation and ex- 

 periment to bring about this result. The doctrine omne 

 vivum ex vivo consorted well with the metaphysical hypo- 

 thesis of an archceus or " vital principle," by means of which 

 Stahl and Paracelsus sought to explain the dynamic pheno- 

 mena manifested by living organisms. In those days when 

 it was the fashion to exjjlain every mysterious group of 



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