352 THE BEGINNINGS OF LIFE. 



in the foot itself, rather than from externally derived 

 germs, 



The occurrence of such growths within closed cavi- 

 ties not exposed to the incidence of external germs, 

 and their not more frequent presence in the ramified 

 air-chambers of birds (where germs might so well lodge 

 and grow), is just as much against the supposition of 

 their external derivation, as the comparatively rare 

 presence of the growths in these cavities militate 

 against their being developments of some of the many 

 minute and invisible germs with which the tissues of 

 higher organisms are believed, by a few persons, to 

 be permeated. This latter view is only countenanced 

 by the occurrence of facts which are thought by others 

 to be equally capable of receiving a totally different 

 explanation. 



During the last fifteen or twenty years, another most 

 important disease, c Pebrine,' has prevailed amongst the 

 silk-worms of France, which, while of the utmost con- 

 sequence commercially, has seemingly been due to the 

 ravages produced by certain extremely minute vesicular 

 parasites usually known as Psorosf ermine organisms 

 concerning whose real nature and affinities systematists 

 are not agreed. Some place them amongst the equally 

 obscure group of Gregarinidse ; whilst others, such as 

 Robin and Balbiani, consider them to be more allied to 

 Diatoms or aberrant forms of Algse. Organisms of a 

 similar nature, though rather more complex in structure, 



