THE ANIMAL IN THE WORLD 



549 



taneous Generation — the origin of living from lifeless 

 matter — and on the other of the rival and now victorious 

 theory of Biogenesis, which maintains the aphorism omne 

 vivum e vivo. Finally, however, it has been demonstrated 

 that organic matter will not develop micro-organisms if, 

 after being properly sterilised by heat, it be placed while 

 it is still hot into a sterilised vessel to which only filtered 

 air has access, or if it be sterilised in such a vessel. 1 The 

 explanation of the appear- 

 ance of such organisms in 

 other circumstances is that 

 they give rise to minute 

 germs or "spores" which 

 are capable of existing in a 

 dried state, and in that state 

 are carried by the air, to 

 germinate when they fall on 

 suitable ground; and that 

 such spores were present 

 either in the substance 

 which putrefies or, if that 

 have been rendered sterile, 

 in the vessel which contains 

 it, or in air which has access 

 to it. The micro-organ- 

 isms are killed both in the 

 organic matter and in the fig. 398A.— A hot-air steriliser.— 

 vessel by sterilisation, and From Muir and Ritchie, 



their germs are filtered out Note the plugs of cotton wool in the 

 from the air which enters as mouths of the vessels - 



the vessel cools. In this, as in every other instance which 

 has been carefully investigated, it is proved to be the case 

 that organised bodies arise only from living bodies of their 

 own kind. The first organised bodies must have arisen 

 from unorganised matter, and it may be that the conditions 

 in which this happened will some day be discovered and 

 reproduced. But at present it is true that living organised 

 bodies are necessary for the reproduction of their kind. 



1 The experiments are usually made with a broth or infusion of meat 

 or hay which is kept in test tubes whose mouths are plugged with 

 cotton wool, put in while the contents are boiling, to serve as a filter. 



