viir r.IOGENESIS AND AHIOGENESIS 253 



the burden of proof that other like forms are 

 generated in a different manner, must rest with 

 the assertor of that proposition. 



To sum up the effect of this long chain of 

 evidence : 



It is demonstrable that a fluid eminently fit for 

 the development of the lowest forms of life, but 

 which contains neither germs, nor any protein 

 compound, gives rise to living things in great 

 abundance if it is exposed to ordinary air ; while 

 no such development takes place, if the air with 

 which it is in contact is mechanically freed from 

 the solid particles which ordinarily float in it, and 

 which may be made visible by appropriate means 



It is demonstrable that the great majority of 

 these particles are destructible by heat, and that 

 some of them are germs, or living particles, capable 

 of giving rise to the same forms of life as those 

 which appear when the fluid is exposed to un- 

 purified air. 



It is demonstrable that inoculation of the ex- 

 perimental fluid with a drop of liquid known to 

 contain living particles gives rise to the same 

 phenomena as exposure to unpurified air. 



And it is further certain that these living 

 particles are so minute that the assumption of 

 their suspension in ordinary air presents not the 

 slightest difficulty. On the contrary, considering 

 their lightness and the wide diffusion of the 

 organisms which produce them, it is impossible to 



